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return true" onmouseout="window.status=''; return true">Other Research</a></li> </ul> </div> <div id="content"> <div class="wrap-content"> <div class="primary-content-2"> <div class="main-info"> <div class="main-info-t"> <div class="main-info-b"> <div class="products"> <div class="title title-sub"> <h1>Aquatic & Fisheries Sexual Differences in Spawning Behaviour and Catchability of Plaice West of Iceland Star-Oddi.com</h1> </div> <div class="content"> <div class="content-bg"> <div class="crumb"> You are here : <a href="/" target="_self" onmouseover="window.status='Home'; return true" onmouseout="window.status=''; return true">Home</a> > <a href="/Home/Aquatic-Fisheries-Research/Aquatic-Fisheries-Research/" target="_self" onmouseover="window.status='Aquatic &amp; Fisheries Research'; return true" onmouseout="window.status=''; return true">Aquatic &amp; Fisheries Research</a> > <a href="/Home/Aquatic-Fisheries-Research/Fish-and-Marine-Animal-Tagging/" target="_self" onmouseover="window.status='Fish and Marine Animal Tagging'; return true" onmouseout="window.status=''; return true">Fish and Marine Animal Tagging</a> > </div> <p>Fisheries Research <br /> Volume 61, Issues 1-3 , March 2003, Pages 57-71<br /> doi:10.1016/S0165-7836(02)00212-6&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</p> <p><br /> NOT TO BE CITED WITHOUT PRIOR REFERENCE TO THE AUTHOR<br /> <br /> <span class="header1">Sexual differences in spawning behaviour and catchability of plaice (Pleuronectes platessa) west of Iceland <br /> </span><br /> <em>by<br /> Jon Solmundsson, , a, b, Hjalti Karlssona and Jonbjorn Palssona <br /> <br /> a Marine Research Institute, P.O. Box 1390, Skulagata 4, 121, Reykjavik, Iceland<br /> b Institute of Biology, University of Iceland, Grensasvegur 12, 108, Reykjavik, Iceland <br /> <br /> Received 27 December 2001;&nbsp; revised 5 September 2002;&nbsp; accepted 18 September 2002. ; Available online 27 November 2002</em>. <br /> <br /> <br /> <span class="header1">Abstract</span><br /> The vertical and horizontal movements of mature plaice (Pleuronectes platessa) on spawning grounds west of Iceland were studied, using conventional anchor tags and electronic data storage tags measuring depth and temperature at specified time intervals. Concurrent to the tagging, information on sex ratios, maturation and spawning was analyzed using data from commercial catches. Males were more active than females during the time of spawning, an increase in swimming activity mainly being observed during the hours around midnight. A higher recapture rate of tagged males than females, indicates a greater catchability of males during the spawning season. Furthermore, males predominated in the catch and it is suggested that this may be explained partly by the observed difference in the behaviour of the sexes. The skewed sex ratios could also be caused by earlier sexual maturation of males, together with males spending more time on the spawning grounds than females. We conclude that sexual differences in behaviour, catchability, age distribution and duration of spawning time of plaice, should be accounted for in catch and survey data sampled from spawning grounds. <br /> <br /> Author Keywords: Spawning behaviour; Plaice; Pleuronectes platessa; Catchability; Data storage tags&nbsp;<br /> &nbsp;<br /> Article Outline<br /> 1. Introduction<br /> 2. Materials and methods<br /> 2.1. Study area<br /> 2.2. Tagging experiments<br /> 2.3. Sampling from commercial catches<br /> 2.4. Data analysis<br /> 3. Results<br /> 3.1. Conventional tag recaptures<br /> 3.2. Depth records<br /> 3.3. Temporal changes in activity<br /> 3.4. Temperature measurements<br /> 3.5. Sex proportions, age composition and maturity<br /> 4. Discussion<br /> Acknowledgements<br /> References<br /> <br /> &nbsp;<br /> <span class="header1">1. Introduction</span><span class="header1"><br /> </span>The catch of demersal marine fish species on the spawning grounds is often biased towards one sex (e.g. McKenzie; Morgan and Lawson), a phenomenon observed for plaice (Pleuronectes platessa) where males usually outnumber females (Hefford and Simpson). The main assumption is that the males occupy the spawning grounds for a longer time than females ( Hefford and Arnold). There may also be other underlying factors, since sex ratios have been found to vary between spawning areas, sections of the same spawning area, and from year to year ( Simpson, 1959). Tagging experiments conducted at spawning grounds have shown higher recapture rates of males than of females ( de Veen, 1964), suggesting that this may be due partly to sexual differences in behaviour ( Beverton, 1964).<br /> <br /> Since the 1970s various electronic methods have been used in the North Sea to observe the behaviour of individual plaice under field conditions. These include acoustic telemetry (Greer; Harden and Metcalfe) and data storage tags (DSTs) ( Metcalfe and Metcalfe). These tags have added valuable information to previous knowledge of plaice behaviour, including migration, based on trawl surveys and conventional tagging experiments (see reviews by Wimpenny (1953) and Harden Jones (1968)). The North Sea plaice exhibits two main patterns of vertical migrations. At the feeding grounds plaice undertake diel vertical movements up the water column at night, whereas during migration, plaice move off the bottom when the tidal stream flows in a favourable direction ( Greer; Harden and Arnold). Although the locations and timing of plaice spawning and migration to and from the spawning grounds are relatively well known ( Wimpenny; Harding; Rijnsdorp; Horwood; Rijnsdorp; Arnold; Ellis and Bromley), the behaviour of male and female plaice on spawning grounds is not. In fact, little is known of the spawning behaviour of most marine demersal fish species on continental shelves ( Hutchings et al., 1999).<br /> <br /> The Icelandic plaice undertakes annual migrations between the spawning and feeding grounds, the main spawning taking place southwest and west of Iceland in March and April, and the main feeding occurring in the south, west and northwest during June&ndash;November (Taning; Sigurdsson and Hjorleifsson). These movements have been studied using conventional tagging methods and catch data from research vessels and the commercial fisheries. However, variations in fish behaviour, which may cause changes in availability and catchability, are not easily detected from conventional tagging or fishery data. The objective of this paper is to examine sexual differences in the spawning behaviour of plaice and how they may affect catchability. This was primarily accomplished by monitoring individual fish with DSTs as well as with conventional tags, supplemented by data on sex ratios and maturation obtained from commercial fisheries. The focus is on plaice spawning in the bay of Breidafjordur, West Iceland.<br /> <br /> <span class="header1">2. Materials and methods</span><br /> 2.1. Study area<br /> Scientific data on plaice spawning in the western area off Iceland are lacking. According to local fishermen, the main spawning takes place at the slope of the northern shelf area in the center of the Breidafjordur bay. As a first approach, we assume the main spawning grounds to be the area covering recaptures of actively spawning plaice in the first year of the tagging studies described here (Fig. 1). This and the surrounding area are characterized by a complex bottom topography, with depths mainly ranging from 50 to 250 m. In most places the bottom has a gradual southward decline, while in others depth is widely variable with shallow banks or ridges on the main slope (Fig. 1; Table 1). Because of this bottom topography, movement of fish along the bottom is likely to involve a change in depth, that can be recorded by electronic DSTs attached to the fish.&nbsp;<br /> <br /> &nbsp;<br /> <br /> <img border="0" alt="" src="/resources/Images/Papers/259_1.jpg" /><br /> Fig. 1. Bottom topography (depth exaggerated) and location of the main spawning area of plaice in Breidafjordur West Iceland, according to recapture sites of actively spawning plaice in the year of tagging (circles). Black and white crosses show the tagging sites in 1997 and 1998, respectively. The distance between the tagging sites is 26.7 km.&nbsp;</p> <p>&nbsp;</p> <p><img border="0" alt="" src="/resources/Images/Papers/260_2a.jpg" /><br /> Table 1. The inclination of the bottom at the plaice spawning grounds in Breidafjordur, estimated using a map of Breidafjordur published by the Royal Danish Hydrographic Office, Copenhagen, 1946 (reproduced and printed by the Icelandic Hydrographic Service, Reykjavik, 1961)a <br /> <br /> From 2&ndash;15 April 1998 near surface temperatures in central Breidafjordur ranged from 1.4 to 3.9 &deg;C over the eastern part of the spawning grounds, and 3.4&ndash;5.3 &deg;C in the western part of the area (Marine Research Institute, unpublished data). Bottom temperatures in Breidafjordur are not available for the years of tagging. It may, however, be assumed that bottom temperatures were similar to near surface temperatures, as the water column near the western shore of Iceland does not become stratified until late April or May (Thordardottir and Stefansson, 1977).<br /> <br /> The tides in Icelandic coastal waters are mainly semi-diurnal (M2) with a period of about 12.5 h. They move in a clockwise direction around Iceland. The difference in height of the sea surface between flood and ebb in Breidafjordur ranges from 3 to 4 m near the coast during average spring tides. Little is known of the direction and velocity of the tidal currents in the study area. In general, the direction of tidal currents on the continental shelf west of Iceland is along isobaths, i.e. north/south in offshore waters and east/west inside fjords (S. Jonsson, Marine Research Institute, pers. commun.).<br /> <br /> <span class="header1">2.2. Tagging experiments<br /> </span>Two tagging experiments were conducted in 1997 and 1998. The first was carried out in the western part of the Breidafjordur spawning area on 5 April 1997 when 700 plaice were tagged with conventional T-Bar anchor tags. The second experiment was conducted in the eastern part of the same spawning area on 29 March 1998 when 613 plaice were tagged with conventional tags of which 62 were also fitted with electronic DSTs. These plaice were captured by Danish seine nets and kept in 660 l deck tanks with running seawater until tagging, 10 min&ndash;2 h later. Wounded, sluggish or apparently lifeless plaice were not tagged. The total length of tagged plaice was measured to the nearest centimetre and the sex and maturity noted when possible, e.g. when eggs or sperm were expelled or ovaries were greatly swollen and extending to the posterior part of the fish. Concurrent to the taggings, length, body mass, age (determined from seasonal rings in otoliths), sex and maturity of an additional 100 plaice were recorded each year. These samples were used to estimate the sex proportions and maturity stages of plaice caught in the area, for comparison with sex ratios of recaptures.<br /> <br /> No commercial fishing was conducted on the spawning grounds for about 3 weeks after the taggings. All recoveries were from the commercial fishery and most tags were returned together with information on recapture place and time. Recapture position was ascertained from the GPS equipment of the fishing vessels. In about 80% of the tag recoveries in April and May the entire fish was returned along with the tags and the recapture information.<br /> <br /> The DSTs (DST 300, Star Oddi, Iceland) are cylindrical, 13 mm in diameter, 46 mm long, weigh 1 g in water, have a memory of 8100 measurements and were programmed to measure both depth (pressure) and temperature at specified time intervals for 6&ndash;12 months. The tags were programmed in two different ways: DST(A)s had repeated 1-week cycles, measuring at 4 h intervals for the first 3 days and at 30 min intervals for the next 4 days. DST(A)s were able to collect data for about 135 days. The depth and temperature range of DST(A)s were pre-programmed at approximately 0&ndash;200 m and -6 to 15 &deg;C, respectively. DST(B)s were programmed to collect data for 337 days, having 2-week cycles and measuring at 4 h intervals for the first 12 days and at 30 min intervals for the next 2 days. The depth and temperature range of DST(B)s were pre-programmed at approximately 0&ndash;400 m and -6 to 15 &deg;C. The resolution for depth measurements cited by the manufacturer is &plusmn;0.8 m for DST(A)s and &plusmn;1.6 m for DST(B)s, and the resolution for temperature is &plusmn;0.1 &deg;C.<br /> <br /> All tags were attached to the dorsal pigmented side anterior to the widest part of the plaice. Two-third of the DSTs were attached by inserting stainless silver wire (0.5 mm in diameter) through two temporarily inserted hollow needles. A silicon ribbon (15 mm&times;63 mm&times;2 mm) was slipped over the wire to minimize skin abrasion from the DST which followed. The wire ends were twisted together above the DST, leaving a slack of approximately 3 mm in the wire to allow for growth. On the blind side of the fish, an oval plastic plate (11 mm&times;50 mm&times;1 mm) was used to minimize erosion by the wire. It took about 1&ndash;2 min to fasten a DST to the plaice by this method. One-third of the DSTs were attached by shooting nylon double sided T-Bar anchor attachments through silver wire loops attached at both ends of the DSTs, into the muscle of the plaice and under the neural spines for a firm grip. A silicon ribbon was also fitted under those DSTs for protection. The tagging of a single plaice by this method took less than 1 min.<br /> <br /> <span class="header1">2.3. Sampling from commercial catches</span><br /> Plaice were sampled from commercial catches taken in the spawning area in 1997 and 1998. These samples were taken at random and mainly came from the Danish seine fisheries. For each sample, total length (to the nearest centimetre), sex and maturity of 100&ndash;300 plaice were recorded, of which 50&ndash;100 were also weighed and otoliths extracted for age determination. The stage of maturation was recorded according a 4-stage classification, modified from Wimpenny (1953) (see Table 2).&nbsp;<br /> <br /> &nbsp;<br /> <img border="0" alt="" src="/resources/Images/Papers/261_table2.jpg" /><br /> Table 2. The classification of maturity stages of plaice used in this study (simplified from Wimpenny (1953), equivalent stages are given in parentheses)&nbsp;<br /> <br /> <br /> <span class="header1">2.4. Data analysis</span><br /> Analysis of depth changes recorded by the DSTs was done only for periods with a measurement frequency of 30 min. Depth changes were calculated as the difference between a given measurement and the next previous measurement. When analyzing plaice activity statistically, depth changes were classified as &quot;no depth change&quot;, &quot;depth change down&quot; (to deeper water) or &quot;depth change up&quot; (to shallower water). Depth changes of 2 m or less were classified as &quot;no depth change&quot; to filter out those caused by tides (change&lt;0.5 m/30 min), sea waves and measurement resolution (0.8&ndash;1.6 m). For individual plaice the number of measurements in each category were calculated for each 3 h time period of the day (e.g. 12:00&ndash;15:00 h) during specified days (e.g. 1&ndash;15 April). The probability of a depth change was similarly calculated as the proportion of all measurements showing depth changes (in either direction) of more than 2 m, this probability being used as an indicator of plaice swimming activity.<br /> <br /> Data on depth changes (y) were analyzed using generalized linear models (GLMs), accommodating a binomial response distribution and a logit link function (McCullagh and Nelder, 1989). Sex (level of activity, s, of each sex) and time (effects of time of day, t, on activity) were set as factors in the analysis, so the full model is of the form:<br /> <br /> <em>y</em>=<img border="0" alt="small alpha, Greek" src="http://www.sciencedirect.com/scidirimg/entities/alpha.gif" />+<img border="0" alt="small beta, Greek" src="http://www.sciencedirect.com/scidirimg/entities/beta.gif" /><sub>s</sub>+<img border="0" alt="small gamma, Greek" src="http://www.sciencedirect.com/scidirimg/entities/gamma.gif" /><sub>t</sub><br /> <br /> <br /> Significance levels (testing whether s=0 and t=0) were tested using a chi-square test. S-Plus was used for statistical analysis.<br /> 3. Results<br /> 3.1. Conventional tag recaptures<br /> Actively spawning plaice were mainly recaptured along or near the slope where the tagging experiments were conducted (Fig. 1 and Fig. 2). The only exception was a milting male recaptured further north on 26 May, at a minimum travel distance of 130 km from the tagging location (Fig. 2).&nbsp;<br /> <br /> <img border="0" alt="" src="/resources/Images/Papers/268_fig2.jpg" /><br /> Fig. 2. The distribution of recaptures from plaice tagging experiments on spawning grounds in Breidafjordur in 1997 and 1998 (Fig. 1). Black dots, open circles and black crosses represent maturity stages 2&ndash;4, respectively (see Table 2). Question marks show recaptures where maturity stage is unknown. The 100 m depth contour is shown. <br /> <br /> <br /> <br /> Although the distribution of recapture sites of the sexes was similar, relatively more males were recaptured on the main spawning grounds (Fig. 2; Table 3). The samples taken during the tagging cruises indicated that the mature plaice tagged were 77.1% males in 1997 and 73.0% in 1998 (Table 3). At the end of May each year (after about 40 days of fishing and 60 days from tagging) the proportion of males among recaptured plaice was larger than in the samples. This holds both for total recaptures and recaptures in the spawning area ( Table 3). In 1998 the sex ratios were significantly altered from those expected by equal recapture rates (chi-square tests, d.f.=1, P&lt;0.01). This was not the case in 1997.&nbsp;<br /> <br /> The recaptures indicate a northerly post-spawning movement to summer feeding grounds (Fig. 2).<br /> <br /> <span class="header1">3.2. Depth records<br /> </span>A total of 28 plaice fitted with DSTs was recaptured: 8 females and 20 males (Table 4). Most plaice returned to the bottom within 4 h after being released and all plaice within 16 h. In the following days the behaviour of the fish did not appear to differ from the behaviour during the next few weeks, judging from the depth plots (Fig. 3). However, when analyzing plaice behaviour, the decision was made to omit the first 3 days after tagging because of possible unnoticed effects of the tagging procedures on the behaviour of the plaice.&nbsp;<br /> <br /> &nbsp;<br /> <img border="0" alt="" src="/resources/Images/Papers/263_table4.jpg" /><br /> Table 4. Information of recaptured plaice fitted with DSTs on spawning grounds in Breidafjordur on 29 March 1998<br /> <br /> <br /> <br /> <br /> <img border="0" alt="" src="/resources/Images/Papers/269_fig3.jpg" /><br /> <br /> Fig. 3. Examples of depth records for mature plaice (females to the left, males to the right) tagged on spawning grounds in Breidafjordur 1998, shown for the period from 29 March to 31 May. Information on each plaice and DST are given in Table 4. Depths are plotted at 4 h intervals. <br /> <br /> <br /> The eight females recaptured, showed variable behaviour at the peak of spawning (in April). In three females depth changes were relatively common, vertical movements often being followed by staying at a constant depth for 1&ndash;3 days (DST 1803, 1870 and 1889, Fig. 3). This may indicate movement along the bottom, up and down the slope, but also off bottom movement followed by settling at a different depth level. Short time changes in depth up to 50 m, appearing as spikes on the plots, were also observed and probably indicate movement off the bottom. In the other five females depth changes in April were rare or almost absent (e.g. DST 1829 and 1861, Fig. 3), indicating little movement. In early May movements had become more frequent, a part of the females possibly migrating at that time according to conventional tag recaptures. In late May there were almost no depth changes (with the exception of DST 1870).<br /> <br /> Males changed depth more frequently than females. Throughout the spawning season most males were frequently changing depth between 90 and 150 m (Fig. 3). Of depth plots from 20 recaptured males fitted with DSTs, 15 showed these pronounced depth oscillations from early April to the middle of May, while five plots were more stable during this period (e.g. DST 1821). By late May vertical movement of males had become less prominent.<br /> <br /> <span class="header1">3.3. Temporal changes in activity</span><br /> The depth occupied by the 10 plaice shown in Fig. 3 during 48 h periods of 10&ndash;12 April and 24&ndash;26 April, indicate that females were mainly lying on the bottom because regular sinusoidal changes in depth due to tidal effects are clearly visible on these depth plots (Fig. 4). DST 1803 and 1870, however, conducted few but large vertical movements in both periods, mainly during the dark hours. In contrast, most males showed considerable motility by frequently changing depth (Fig. 4). The change of depth occurred during the hours of darkness while during the day males were mainly lying on the bottom, with only one exception (DST 1834 on 11 April). The observed increase in activity was initiated either by upward or downward movements, followed by irregular depth oscillations during the night.&nbsp;<br /> <br /> &nbsp;<br /> <br /> </p> <p><img border="0" alt="" src="/resources/Images/Papers/270_fig4.jpg" /><br /> Fig. 4. Depth records for five mature female plaice (to the left) and five mature male plaice (to the right) during 48 h periods from 10 to 12 April 1998 (upper figures) and 22&ndash;24 April 1998 (lower figures). The time of slack water in Breidafjordur is shown by L (low slack water) and H (high slack water). The bars above each figure denote the time between sunset and sunrise (Almanac for Iceland 1998, University of Iceland). Depths are plotted at 30 min intervals. <br /> <br /> <br /> <br /> To estimate swimming activity, all depth changes of more than 2 m (up or down) were used. The resulting probability of depth change for all recaptured plaice fitted with DSTs clearly reflects differences in activity between the sexes, especially in April to early May (Fig. 5). Males showed a clear diel rhythm in swimming activity, being most active in the time period between 21:00 and 03:00 h. In contrast, females showed relatively little activity throughout the 24 h, but the activity of females was greater in May than in April (Fig. 5). The probability of a depth change was significantly dependent upon sex and time of the day, according to the GLM models ( Table 5). These factors explained a considerable part of the total deviance in the GLM models in April (44&ndash;51%), but only a small part in May (5&ndash;20%).&nbsp;<br /> <br /> &nbsp;<br /> <br /> <img border="0" alt="" src="/resources/Images/Papers/271_fig5.jpg" /><br /> Fig. 5. The estimated probability of a depth change of mature male plaice (upper figures) and female plaice (lower figures) by time of day in April and May (see methods). The height of a box is equal to the interquartile distance of the data (IQD: the difference between the third and the first quartile) and the horizontal bar inside the box gives the median. The whiskers extend to the extreme values of the data or a distance 1.5&times;IQD, whichever is less. Horizontal lines outside the whiskers represent outliers. <br /> <br /> <br /> <img border="0" alt="" src="/resources/Images/Papers/264_table5.jpg" /><br /> Table 5. Analysis of deviance for GLMs fitted to data on depth changes of plaice, using &quot;sex&quot; and &quot;time of day&quot; as factorsa <br /> <br /> <br /> <br /> The frequency of occurrence of four size categories of depth changes during various stages of the tidal cycle is shown in Table 6. Data collected in May were not divided into tidal categories, as some plaice had probably migrated out of the spawning area (Fig. 2). As already shown, depth changes of males were more frequent than those of females, especially during darkness. This nocturnal swimming activity of males was elevated both at flood and ebb tides as well as during slack water ( Table 6; Fig. 6). Depth changes of more than 10 m were relatively rare during the day (average of 0.3&ndash;1.0%), but more frequent during the night (1.6&ndash;5.8%). The largest depth change observed in a 30 min interval was an 82 m descent of DST 1870 on 12 April (Fig. 4).&nbsp;<br /> <br /> &nbsp;<br /> <img border="0" alt="" src="/resources/Images/Papers/265_table6.jpg" /><br /> Table 6. Depth changes of plaice (between successive 30 min measurements) presented as frequency of occurrence (%) of four size categories of depth changes during various stages of the tidal cycle in central Breidafjordur (LS: low slack water, HS: high slack water, F: flood tide, E: ebb tide) during day and night</p> <p><br /> <br /> <img border="0" alt="" src="/resources/Images/Papers/272_fig6.jpg" /><br /> Fig. 6. The frequency of occurrence (%) of depth changes more than 2 m for mature male plaice during various stages of the tidal cycle in April. D: day=06:00&ndash;18:00 h, N: night=18:00&ndash;06:00 h. Plot definitions as in Fig. 5. For all tidal stages, the mean was significantly higher during the night (Wilcoxon rank sum test, P&lt;0.001). <br /> <br /> <br /> <br /> <span class="header1">3.4. Temperature measurements</span><br /> The daily mean temperature recorded by DSTs increased from 1.9 &deg;C (n=28, SD=0.2) on 1 April to 4.1 &deg;C (n=25, SD=0.5) on 1 May, reaching 5.5 &deg;C (n=21, SD=0.6) by 31 May. Temperature readings from two plaice differed markedly from this. The location of a male plaice (DST 1822) probably differed from other plaice, according to temperature measurements of 5.6&ndash;5.9 &deg;C in the second half of April. A large female plaice (DST 1870) probably left the Breidafjordur area in early May, according to ambient temperature readings of about 3 &deg;C, and movement to depths about 400 m in late May.<br /> <br /> <span class="header1">3.5. Sex proportions, age composition and maturity</span><br /> Mature plaice predominated in the catch samples taken in 1997 and 1998, immatures being only 2&ndash;18%. In all samples the sex proportions of mature plaice were biased towards males (&gt;73%) and differed significantly from the expected equal ratio (chi-square test, d.f.=1, P&lt;0.001, Table 7).&nbsp;<br /> <br /> &nbsp;<br /> <img border="0" alt="" src="/resources/Images/Papers/266_table7.jpg" /><br /> Table 7. Samples taken from commercial and research (*=tagging cruise) catch of plaice on spawning grounds in Breidafjordur in 1997 and 1998<br /> <br /> <br /> The majority of mature plaice in the catch in 1997 were 5&ndash;9 years old, the 1990 year-class being most prominent both among males and females (Table 8). The proportion of mature plaice younger than 7 years was, however, higher (42%) for males as compared to females (8%). Males predominated in all age-classes although not significantly for 8 and 9-year-old plaice (Table 8).&nbsp;<br /> <br /> &nbsp;<br /> <img border="0" alt="" src="/resources/Images/Papers/267_table8.jpg" /><br /> Table 8. The age distribution of mature male (n=288) and female (n=80) plaice and the percentage of males in each age class, from the commercial catch of plaice on spawning grounds in Breidafjordur in 1997<br /> <br /> <br /> When port sampling started on 24 February 1997, 50% of mature males were already milting but females had apparently not begun to shed eggs (Table 7). On 18 March the proportion of milting males had increased to 76% and on 5 April all mature males were milting while 50% of females were running. The first females apparently finished spawning between 5 and 28 April while spent males were first observed on 8 May. In late May, fishing activity at the spawning area was greatly reduced because most plaice had migrated out of the area. Thus, the spawning period in Breidafjordur in 1997 started at the beginning of March, reached a peak in April and was mostly finished in late May.<br /> <br /> <span class="header1">4. Discussion</span><br /> The nocturnal swimming activity of male plaice observed in the present study is unlikely to be related to feeding behaviour, as the feeding of mature plaice sampled was negligible according to a visual inspection of stomach contents. Similar conclusions were reported for plaice spawning in the North Sea (Rijnsdorp, 1989). Furthermore, plaice are visual feeders and feeding is usually restricted to the daylight hours ( de and Hempel). In the southern North Sea, the spawning of plaice has a diel rhythm, with most spawning occurring between 18:00 and 07:00 h and peaking around midnight (Simpson and Nichols). This timing of spawning coincides with the movement of males observed during the spawning season in Breidafjordur. We therefore believe that the increased activity of males during nighttime, independent of the tidal cycle, is associated with reproductive behaviour. To our knowledge, this is the first study to measure sexual differences in the spawning behaviour of plaice under field conditions.<br /> <br /> The inclination of the bottom on the spawning grounds in Breidafjordur is greatly variable, usually ranging from 1 to 84 m/1000 m horizontally (Table 1). The speed of 41&ndash;47 cm plaice swimming close to the bottom has been found to range between 2 and 62 cm/s over the ground (plaice 1, 2 and 7 in Greer Walker et al., 1978). A plaice swimming at an average speed of 62 cm/s can cover a horizontal distance of 1.1 km in 30 min, and could therefore experience a depth change of up to 92 m in the study area in Breidafjordur. Assuming a moderate swimming speed of 21&ndash;41 cm/s (equivalent to 0.5&ndash;1.0 body length/s for the average plaice recaptured in the present study) gives a maximum estimate of 32&ndash;62 m depth change per 30 min. The angle of ascent and descent of plaice moving vertically in mid-water has been shown to be small (2&ndash;8&deg;), and the rate of change of depth usually between 0.7 and 2.4 m/min (Greer Walker et al., 1978), equivalent to 20&ndash;72 m between successive 30 min measurements. It is therefore difficult to determine from the rates of depth changes in the present study, whether they originate from movements along the bottom or mid-water excursions. However, short term changes in depth appearing as spikes on the depth plots presumably indicate some off bottom migration, although such movements were relatively rare. An increase in activity in the present study was often initiated by a downward movement, whereas upward movements are to be expected by plaice moving directly to mid-water. The pronounced swimming activity of males is therefore interpreted mainly as movement along or near the bottom between different depth levels. It is reasonable to expect comparable movement at isodepths, although not recorded by DSTs.<br /> <br /> It seems that during spawning, female plaice are relatively stationary on the spawning grounds, because any significant movement within the area is likely to involve a change of depth. Observation of plaice spawning in tanks have shown that both sexes tend to be relatively active shortly before actual spawning. When ready to shed eggs the female settles and stays in close contact with the bottom while the male swims actively above or near the bottom (Beverton, 1964). Male fish of captive sole (Solea solea) have been shown to be much more active than females prior to spawning (Baynes et al., 1994). This behaviour may suggest female mate choice and male&ndash;male competition as shown for Atlantic cod (Gadus morhua), where interaction between sexes was dominated by circling of benthic reproductive females by males (Hutchings et al., 1999).<br /> <br /> We observed higher proportions of mature males than females in the commercial catch of younger year-classes, presumably because male plaice recruit to spawning stocks at an earlier age than females (Wimpenny and Rijnsdorp). In contrast to earlier studies ( Hefford and Simpson) we also observed more males among older plaice. On the other hand, catches of plaice 10 years and older on summer feeding grounds west of Iceland consist mainly of females (unpublished data).<br /> <br /> Plaice catches at spawning areas have shown some diel variation, night catches usually being greater than day catches (Woodhead and Simpson). Hefford (1909) found greater night catches on spawning grounds to be due to an increased catch of males, while the number of females decreased, indicating some underlying sex based differences in behaviour. The swimming activity of males, revealed by the present study, may increase their vulnerability, as a high level of swimming activity near the bottom is likely to cause flatfishes to be more readily caught than those lying on the bottom, perhaps partially buried ( Beamish and Woodhead). In line with our results, tagging experiments of spawning plaice in the North Sea indicate a greater catchability of males ( de Veen, 1964). Furthermore, Beverton (1964) found the total fishing mortality coefficient of North Sea plaice to be 50&ndash;150% higher for males than females, and greater catchability of males particularly marked among mature fish at spawning time.<br /> <br /> Together with earlier maturation and greater vulnerability, males may outnumber females because of a longer spawning duration. There is evidence that male plaice complete their pre-spawning migration earlier than females, and return to feeding grounds later (Arnold and Metcalfe, 1996). The mean spawning time duration of the North Sea plaice population has been estimated at least 11 weeks for males and about 5 weeks for females ( Rijnsdorp, 1989). The present study also shows the milting period of males to greatly exceed the period in which running females are found. The time spent on the spawning grounds cannot, however, be derived directly from the relative duration of active spawning, as females reach the spawning grounds in the late ripening stage in contrast to many males that arrive there already milting ( Arnold and Metcalfe, 1996).<br /> <br /> The temperature data from all DSTs in the former half of April (range: 1.2&ndash;5.6 &deg;C), compared to contemporary data on sea water temperature in the spawning area (range: 1.4&ndash;5.3 &deg;C), do not indicate a post-release migration of DST tagged plaice outside the spawning grounds. The ambient 5.6&ndash;5.9 &deg;C temperature of DST 1822 during the second half of April, indicates a movement from the main aggregation of the DST tagged plaice. This, however, does not necessarily mean that this fish migrated outside the spawning area, as temperatures of 5.3 &deg;C were measured at the westernmost part of the spawning area already in early April.<br /> <br /> A question of some concern is whether the behaviour of the DST tagged plaice is somehow different from the untagged population. The plaice is a robust fish and it has been shown that other electronic tags, similar in size but heavier than DSTs used in this study, are unlikely to affect the plaice significantly (Arnold and Holford, 1978). Plaice fitted with DSTs migrated to the same areas as conventionally tagged plaice and we did not observe extra losses of fish because of the DST tagging method. Furthermore, recaptured plaice appeared to be in normal condition, judging by body condition, presence of food in stomachs of plaice from feeding areas and usually little skin erosion underneath the tag.<br /> <br /> Regular sinusoidal changes in depth, resulting from rise and fall of the tides, appear on the tracks illustrated in Fig. 3 and Fig. 4. These depth changes give direct measurements of the tidal range and the times of high and low water; information that can be used, together with measurements of the ambient depth and temperature, to estimate the geographic location of the fish ( Metcalfe and Arnold, 1997). This, however, requires detailed information on the time of high and/or low water and tidal range in the area where the fish travels, as well as bottom temperature measurements. At present, detailed information of this kind are not available for the area studied, making such estimates of geographic locations inaccurate.<br /> <br /> In conclusion, our results reveal that males are more active than females during spawning, the males probably searching for a ready mate, while females apparently spend more time lying on the bottom. This may render males more vulnerable both to moving and stationary fishing gear. The predominance of males in catches on spawning grounds may further be increased by males reaching sexual maturity earlier than females, and their longer stay in the spawning area. If not accounted for in catch and survey data collected on spawning grounds, these sex based differences could affect population statistics by overestimating the proportion of younger fish (males) in the spawning stock.&nbsp;<br /> &nbsp;<br /> <br /> <span class="header1">Acknowledgements</span><br /> We are grateful to many of our colleagues at MRI for various advice and assistance. We would also like to thank K. Lilliendahl, S. Walsh and several anonymous reviewers for constructive comments on earlier versions of this paper, and the staff at Star Oddi for their co-operation. L. Taylor and G. Stefansson advised on statistical analysis and O.P. Olafsson made helpful comments on the text. Finally, we are sincerely grateful to fishermen participating in cruises and returning tagged plaice. This project was supported by Lydveldissjodur (Icelandic Republic Fund, 1994&ndash;1999).&nbsp;<br /> &nbsp;<br /> <br /> <span class="header1">References</span><br /> Arnold and Holford, 1978. G.P. Arnold and B.H. Holford , The physical effects of an acoustic tag on the swimming performance of plaice and cod. J. Cons. Int. Explor. Mer. 38 (1978), pp. 189&ndash;200.<br /> <br /> Arnold and Holford, 1995. G.P. Arnold and B.H. Holford , A computer simulation model for predicting rates and scales of movement of demersal fish on the European continental shelf. ICES J. Mar. Sci. 52 (1995), pp. 981&ndash;990. Abstract | PDF (415 K) <br /> <br /> Arnold and Metcalfe, 1996. G.P. Arnold and J.D. Metcalfe , Seasonal migrations of plaice (Pleuronectes platessa) through the Dover Strait. Mar. Biol. 127 (1996), pp. 151&ndash;160. Abstract-OceanBase | Abstract-GEOBASE&nbsp;&nbsp; <br /> <br /> Baynes et al., 1994. S.M. Baynes, B.R. Howell, T.W. Beard and J.D. Hallam , A description of spawning behaviour of captive Dover sole, Solea solea (L.). Neth. J. Sea Res. 32 (1994), pp. 271&ndash;275. Abstract <br /> <br /> Beamish, 1966. F.W.H. Beamish , Vertical migration by demersal fish in the northwest Atlantic. J. Fish. Res. Bd. Can. 23 (1966), pp. 109&ndash;139.<br /> <br /> Beverton, 1964. R.J.H. Beverton , Differential catchability of male and female plaice in the North Sea and its effect on estimates of stock abundance. Rapp. P.-V. Run. Cons. Int. Explor. Mer. 155 (1964), pp. 103&ndash;112.<br /> <br /> Bromley, 2000. P.J. Bromley , Growth, sexual maturation and spawning in central North Sea plaice (Pleuronectes platessa L.), and the generation of maturity ogives from commercial catch data. J. Sea Res. 44 (2000), pp. 27&ndash;43. SummaryPlus | Full Text + Links | PDF (558 K) <br /> <br /> de Groot, 1964. S.J. de Groot , Diurnal activity and feeding habits of plaice. Rapp. P.-V. Run. Cons. Int. Explor. Mer. 155 (1964), pp. 48&ndash;51.<br /> <br /> de Veen, 1964. J.F. de Veen , On the merits of sampling spawning fish for estimating the relative abundance of different year-classes in plaice. Rapp. P.-V. Run. Cons. Int. Explor. Mer. 155 (1964), pp. 94&ndash;98.<br /> <br /> Ellis and Nash, 1997. T. Ellis and R.D.M. Nash , Spawning of plaice Pleuronectes platessa L. around the Isle of Man, Irish Sea. ICES J. Mar. Sci. 54 (1997), pp. 84&ndash;92. Abstract | PDF (359 K) <br /> <br /> Greer Walker et al., 1978. M. Greer Walker, F.R. Harden Jones and G.P. Arnold , The movements of plaice (Pleuronectes platessa L.) tracked in the open sea. J. Cons. Int. Explor. Mer. 38 (1978), pp. 58&ndash;86.<br /> <br /> Harden Jones, 1968. Harden Jones, F.R., 1968. Fish Migration. Arnold, London.<br /> <br /> Harden Jones et al., 1979. F.R. Harden Jones, G.P. Arnold, M. Greer Walker and P. Scholes , Selective tidal stream transport and the migration of plaice (Pleuronectes platessa L.) in the southern North Sea. J. Cons. Int. Explor. Mer. 38 (1979), pp. 331&ndash;337.<br /> <br /> Harding et al., 1978. D. Harding, J.H. Nichols and D.S. Tungate , The spawning of plaice (Pleuronectes platessa L.) in the southern North Sea and English Channel. Rapp. P.-V. Run. Cons. Int. Explor. Mer. 172 (1978), pp. 102&ndash;113.<br /> <br /> Hefford, 1909. A.E. Hefford , The proportionate distribution of the sexes of plaice in the North Sea. Rapp. P.-V. Run. Cons. Int. Explor. Mer. 11 (1909), pp. 135&ndash;176.<br /> <br /> Hefford, 1916. Hefford, A.E., 1916. Report on sexual differentiation in the biology and distribution of plaice in the North Sea. Fish. Invest. Lond. Ser. 2 3 (2).<br /> <br /> Hempel, 1964. G. Hempel , Diurnal variations in catch, feeding and swimming activity of plaice (Pleuronectes platessa L.). Rapp. P.-V. Run. Cons. Int. Explor. Mer. 155 (1964), pp. 58&ndash;64.<br /> <br /> Hjorleifsson and Palsson, 2001. E. Hjrleifsson and J. Plsson , Settlement, growth and mortality of 0-group plaice (Pleuronectes platessa) in Icelandic waters. J. Sea Res. 45 (2001), pp. 321&ndash;324. SummaryPlus | Full Text + Links | PDF (94 K) <br /> <br /> Horwood, 1993. J.W. Horwood , Fecundity and biomass of plaice (Pleuronectes platessa L.) in the northern Celtic Sea. ICES J. Mar. Sci. 50 (1993), pp. 315&ndash;323. Abstract <br /> <br /> Hutchings et al., 1999. J.A. Hutchings, T.D. Bishop and C.R. McGregor-Shaw , Spawning behaviour of Atlantic cod, Gadus morhua: evidence of mate competition and mate choice in a broadcast spawner. Can. J. Fish. Aquat. Sci. 56 (1999), pp. 97&ndash;104. Abstract-GEOBASE&nbsp;&nbsp; | Full Text via CrossRef<br /> <br /> Lawson and Rose, 2000. G.L. Lawson and G.A. Rose , Small-scale spatial and temporal patterns in spawning of Atlantic cod (Gadus morhua) in coastal Newfoundland waters. Can. J. Fish. Aquat. Sci. 57 (2000), pp. 1011&ndash;1024. Abstract-OceanBase | Abstract-BIOTECHNOBASE | Abstract-GEOBASE | Abstract-Elsevier BIOBASE&nbsp;&nbsp; | Full Text via CrossRef<br /> <br /> McCullagh and Nelder, 1989. McCullagh, P., Nelder, J.A., 1989. Generalized Linear Models. Chapman &amp; Hall, London.<br /> <br /> McKenzie, 1940. R.A. McKenzie , Nova Scotian autumn cod spawning. J. Fish. Res. Bd. Can. 5 (1940), pp. 105&ndash;120.<br /> <br /> Metcalfe and Arnold, 1997. J.D. Metcalfe and G.P. Arnold , Tracking fish with electronic tags. Nature 387 (1997), pp. 665&ndash;666. Abstract-EMBASE&nbsp;&nbsp; | Full Text via CrossRef<br /> <br /> Metcalfe et al., 1993. J.D. Metcalfe, B.H. Holford and G.P. Arnold , Orientation of plaice (Pleuronectes platessa) in the open sea: evidence for the use of external directional clues. Mar. Biol. 117 (1993), pp. 559&ndash;566. Abstract-OceanBase | Abstract-Elsevier BIOBASE | Abstract-GEOBASE&nbsp;&nbsp; <br /> <br /> Metcalfe et al., 1994. Metcalfe, J.D., Arnold, G.P., Holford, B.H., 1994. The migratory behaviour of plaice in the North Sea as revealed by data storage tags. ICES CM 1994/Mini 11.<br /> <br /> Morgan and Trippel, 1996. M.J. Morgan and E.A. Trippel , Skewed sex ratios in spawning shoals of Atlantic cod (Gadus morhua). ICES J. Mar. Sci. 53 (1996), pp. 820&ndash;826. Abstract | PDF (303 K) <br /> <br /> Nichols, 1989. J.H. Nichols , The diurnal rhythm in spawning of plaice (Pleuronectes platessa L.) in the southern North Sea. J. Cons. Int. Explor. Mer. 45 (1989), pp. 277&ndash;283. Abstract-GEOBASE&nbsp;&nbsp; <br /> <br /> Rijnsdorp, 1989. A.D. Rijnsdorp , Maturation of male and female North Sea plaice (Pleuronectes platessa L.). J. Cons. Int. Explor. Mer. 46 (1989), pp. 35&ndash;51. Abstract-GEOBASE&nbsp;&nbsp; <br /> <br /> Rijnsdorp and Pastoors, 1995. A.D. Rijnsdorp and M.A. Pastoors , Modelling the spatial dynamics and fisheries of North Sea plaice (Pleuronectes platessa L.) based on tagging data. ICES J. Mar. Sci. 52 (1995), pp. 963&ndash;980. Abstract | PDF (721 K) <br /> <br /> Sigurdsson, 1989. A. Sigurdsson , Skarkolamerkingar vid Island arin 1953&ndash;1965 (Tagging experiments of plaice in Icelandic waters 1953&ndash;1965). Hafrannsoknir 39 (1989), pp. 5&ndash;24 (In Icelandic) .<br /> <br /> Simpson, 1959. Simpson, A.C., 1959. The spawning of the plaice (Pleuronectes platessa) in the North Sea. Fish. Invest. Lond. Ser. 2 22 (7).<br /> <br /> Simpson, 1971. A.C. Simpson , Diel spawning behaviour in populations of plaice, dab, sprat and pilchard. J. Cons. Int. Explor. Mer. 34 (1971), pp. 58&ndash;64.<br /> <br /> Taning, 1929. .V. Tning , Plaice investigations in Icelandic waters. Rapp. P.-V. Run. Cons. Int. Explor. Mer. 57 (1929), pp. 1&ndash;134.<br /> <br /> Thordardottir and Stefansson, 1977. Thordardottir, Th., Stefansson, U., 1977. Productivity in relation to environmental variables in the Faxafloi region 1966&ndash;1967. ICES CM 1977/L:34.<br /> <br /> Wimpenny, 1953. Wimpenny, R.S., 1953. The Plaice. Arnold, London.<br /> <br /> Woodhead, 1966. P.M.J. Woodhead , The behaviour of fish in relation to light in the sea. Oceanogr. Mar. Biol. Ann. Rev. 4 (1966), pp. 337&ndash;403. <br /> &nbsp;<br /> <br /> &nbsp;Corresponding author. Present address: Marine Research Institute, P.O. Box 1390, Skulagata 4, 121 Reykjavik, Iceland. 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