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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 Size Specific Time and Duration of Spawning of Cod in Icelandic Waters 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>Thorsteinsson, V. and Matreinsdottir, G. 1998. Size specific time and duration of spawning of cod (Gadus morhua) <br /> in Icelandic waters. International Council for the Exploration of the Sea. C.M. 1998/DD-5. 18 p.<br /> &nbsp;<br /> NOT TO BE CITED WITHOUT PRIOR REFERENCE TO THE AUTHOR<br /> <br /> International Council for the&nbsp;Exploration of the Sea&nbsp;ICES CM 1998/DD:5<br /> Theme session on&nbsp;Spawning and recruitment<br /> &nbsp;<br /> <br /> <span class="header1">Size specific time and duration of spawning of cod (Gadus morhua) in Icelandic waters</span><br /> &nbsp;<br /> <br /> <em>by<br /> <br /> Vilhjalmur Thorsteinsson and Gudrun Marteinssdottir<br /> Marine Research Institute, <br /> Sklagata 4, 121 Reykjavk Iceland.<br /> </em><br /> <br /> <span class="header1">ABSTRACT</span><br /> <br /> The time and duration of spawning in Icelandic cod based on the frequency of maturity stages was compared with the time spent on the spawning grounds based on data from Data Storage Tags (DSTs). Tagging with DSTs was performed in the spawning season on the main spawning grounds, off the Southwest coast of Iceland, from 1995 to 1997. Recaptures provided information on depth and temperature in the ambient environment of the fish during time periods from a few hours up to more than a year. The arrival and duration of stay by the fish on the spawning grounds was documented by the DSTs, shown by a distinct change in depth profiles. Time and duration of spawning based on maturity stages and the information from the tags are analysed with respect to size of males and females.<br /> <br /> Keywords: Cod, Gadus morhua, tagging, data storage tags, spawning<br /> <br /> <span class="header1">INTRODUCTION</span><br /> <br /> Cod spawn over a protracted time period that may extend over several weeks or even months (Kjesbu, 1989; 1990; Chambers and Waiwood, 1996). In Iceland, the spawning season typically extends from mid March to mid May (Jonsson, 1982). Recent studies on the time and duration of spawning have indicated that larger females start to spawn earlier in the season and spawn over a longer time period than smaller females (Marteinsdottir and Petursdottir, 1995; Marteinsdottir and Steinarsson, 1996). Spawning among the different size classes appears to be either terminated at a similar time (Marteinsdottir and Steinarsson, 1996) or terminated later in the season among the smaller females (Marteinsdottir and Petursdottir, 1995). Extended spawning season of cod-stocks is achieved in part by repeated spawning by individual females (Kjesbu, 1994; Hislop, et al., 1984) and is likely to reduce the potential for mis-match between larval occurrence and favourable environmental conditions (Mertz and Myers, 1994). The duration of the spawning season may also be extended by asynchronisity of spawning among cod of different ages or sizes. Although the extended spawning period of the larger females (6-8 weeks; Marteinsdottir and Steinarsson, 1996) in comparisons to shorter time period of the smaller females (2-3 weeks), most likely reflects size specific duration of spawning among individuals, the duration of spawning has not been analysed at the individual level.<br /> <br /> A variety of electronic tags have been used in research of fish behaviour since the use of these techniques started in 1956 (Arnold et al 1994a; Baras 1991,1996; Metcalfe et al 1992). Over the last few years electronic Data Storage Tags (DSTs) have become increasingly important. These have been used to study migration, both horizontal and vertical, availability and accessibility of fish stocks and vulnerability to fishing-gear in commercial fishery or surveys (Arnold et al 1997; God &amp; Michalsen 1997; Metcalfe et al 1994; Thorsteinsson 1995).<br /> <br /> As a part of a large tagging program on sexually mature cod on spawning-grounds, cod have been tagged with DSTs in order to study post spawning behaviour in relation to different spawning grounds in Icelandic waters (Thorsteinsson, 1995). Preliminary analysis of data recovered from these tags have indicated that information on the time and duration spent by cod on the main spawning sites off the south-and south-west coast of Iceland may be obtained based on vertical migration patterns, characteristic depth and temperature at the spawning sites during the spawning season.<br /> <br /> Using the background information of time and duration of spawning based on recordings of maturity stages by sampling commercial catches at regular intervals during the spawning season, the present study explores the possibilities of using time series from DSTs to analyse the time spent at the spawning grounds by individual cod.<br /> &nbsp;<br /> <br /> <span class="header1">MATERIALS AND METHODS</span><br /> <br /> Time and duration of spawning based on maturity stages.<br /> <br /> Information on time and duration of spawning on the main spawning grounds were obtained from data collections described in Marteinsdottir and Steinarsson (1996) and Marteinsdottir et al. (in prep). Samples were obtained from area&nbsp;3 (fig.1)&nbsp;during research cruises and from commercial sources, several times every week during the spawning periods. All sampled cod were assigned to four maturity stages (Immature, maturing, spawning and spent; Powels, 1958) where spawning females contained hydrated ovaries within their ovaries and spawning males were characterized by freely running milt.<br /> <br /> <span class="header1">Tagging of cod</span><br /> <br /> Cod were tagged on the main spawning grounds Southwest of Iceland in the coastal area (40-70 m deep; area 3.1 and 3.2 in fig.1) and in the off shore area further out on the bank (95-200 m deep; area 3.4 and 3.5 in fig.1). Cod for tagging were collected from commercial gill-nets with mesh sizes ranging from 150 to 230 mm (stretched diagonal)<br /> <br /> The electronic data tags used were produced by Star-Oddi ehf, Iceland. Two types were used, DST 200 and DST 300. The DST 200 is a cylinder with dimension 18 x 48 mm, programmed to take measurements alternating between an interval of 4 hours during 6 days and an interval of 40 minutes during the next 24 hours. Starting time was 1 hour. Depth range was 0 to 420 m, resolution &plusmn; 2 m. Temperature range was -2 to + 20&deg;C, resolution 0.1 &deg;C. The interval program used allowed for a maximum active time of 390 days with a maximum of 4050 records on temperature and depth.<br /> <br /> The DST 300, which is a 13 x 46mm cylinder, records temperature, depth and tilt of the fish. In 1997, the tags were programmed to record measurements every 6 hours during a period of 6 days and followed by a period of recordings every 60 minutes during the next 24 hours. Starting time was 1 hour. Depth range was 0 to 700 m. Resolution &plusmn; 2 m. Temperature range was -2 to + 20&deg;C &plusmn; 0.1 &deg;C.</p> <p><br /> The cod used for tagging were transferred to a holding tank immediately after being released form the nets. Only those cod that were in good condition and appeared to be able to control their depth in the holding tank (i.e., were active and swam near the bottom of the holding tank) were used in the tagging experiment. The DSTs were implanted in to the peritoneal cavity and secured in place by surgical sutures on a line midway between the cloaca and the isthmus (Petersen &amp; Andersen 1985; Thorsteinsson unpublished results). The DST200 was secured in place by one surgical suture from the inside of the cavity. The DST300, which had a sensor for tilt, had to be secured in two places with its longitudinal axis parallel to the longitudinal axis of the fish. For external identification of cod with a DST, a 15 cm spaghetti tag was fastened to one end of the DST. The spaghetti tag emerged to the exterior through a hole in the body cavity. <br /> <br /> Surgical sutures used were ETHICON, reverse cutting 40 mm, CP, 9321; DEXON II, C-12, 48mm, 9867-71. For sterilization, Superfos Biosector a/s, IDU-SCRUB was used to clean instruments and the tagging saddle. The saddle for surgery was made of wood and soaked in sterilising media during the operation, Flessa Needle, Hose with running seawater (ample but slow running), Cloth to wrap around the head of the fish during the operation, Measuring board (ICES standard).<br /> <br /> For more detailed account of methods see the CATAG web-side&nbsp;<a href="http://www.hafro.is/catag/">www.hafro.is/catag</a><br /> <span class="header1">Results</span><br /> <br /> Time and duration of spawning based on maturity stages<br /> <br /> A total of 8928 cod were sampled for maturity stages in 1996 and 2806 in 1997 (see table 1). Less effort was directed towards sampling in 1997. In addition, seasonally bad weather during the spawning season in 1997 prevented sampling on several occasions. As a result, information on spawners are missing from several time intervals during that year.<br /> <br /> Table 1. Sampling of cod for maturity stages<br /> </p> <table border="1" cellspacing="1" cellpadding="2" width="100%"> <tbody> <tr> <td valign="middle" align="left">&nbsp;Year</td> <td valign="middle" align="left">&nbsp;No of males</td> <td valign="middle" align="left">&nbsp;No of females</td> <td valign="middle" align="left">&nbsp;Total</td> </tr> <tr> <td valign="middle" align="left">&nbsp;1996</td> <td valign="middle" align="left">&nbsp;&nbsp;5318</td> <td valign="middle" align="left">&nbsp;3610</td> <td valign="middle" align="left">&nbsp;&nbsp;8928</td> </tr> <tr> <td valign="middle" align="left">&nbsp;1997</td> <td valign="middle" align="left">&nbsp;&nbsp;1101</td> <td valign="middle" align="left">&nbsp;1705</td> <td valign="middle" align="left">&nbsp;&nbsp;&nbsp;2806</td> </tr> </tbody> </table> <p>In 1996, the spawning season started in middle of march and the first spawning males and females were captured in week 10 (fig.2). Information on the start of spawning is less conclusive in 1997, however spawning among females had, started in week 11 (fig.3).<br /> <br /> Larger females appeared to start spawning earlier and spawn over a longer time period than the smaller females. This was especially true in 1996 where spawning females &gt; 100 cm were captured in great numbers in week 12 through 16 (fig.2). Spawning among females 76-100 cm long was greatest in week 15 and 16 and spawning of females &lt; 76 cm was greatest in week 16 and 17. Less effort was expressed by the smaller females than by the larger females where less than 50% of females &lt; 76 cm were spawning in each time interval. Differences in the time of start of spawning among female size classes were less obvious in 1997, partly due to lack of data. For example, spawning among the smallest females was greater in 1997 than in 1996. More than 60-80 % of the females captured in week 15-18 were spawning in 1997 while less than 50 % of females &lt; 76 cm were spawning during same time period in 1996 (fig.2 and 3).<br /> <br /> Time and duration of spawning among males appeared to be much less size specific. Spawning of males started with a similar effort among all size classes in 1996. Furthermore, the 1997 data, although limited, did not indicate great differences in start of spawning among male size classes (fig.3).<br /> <br /> Termination of spawning did not appear to be particularly size related. In 1996 the spawning season was terminated in week 18, both among males and females and among all size classes. The end of spawning was more variable in 1997. For example, more than 50% of males &gt; 100 cm and males and females &lt; 76 were still spawning in week 18.&nbsp;<br /> <br /> &nbsp;<span class="header1">Tagging of cod</span><br /> <br /> In the area where this study was conducted (area 3 ing fig.1), 398 cod have been tagged with DSTs during the time period from 1995 to 1998. Until June 1998, 125 DSTs were recovered or 31%. The recaptures of DSTs from the tagging experiments in 1996 and 1997 are used in the present study. These tagging experiments involved also tagging with conventional tags (Thorsteinsson 1995), however those results have been omitted in this paper. The time at liberty (release until recapture) extended from a few days to three years. Duration of measurements in DST tags lasted from few hours to a maximum of 391 days depending on time at recapture or data storage capacity of the DSTs. <br /> <br /> <span class="header1">Identification of the spawning period based on DST&rsquo;s depth profiles<br /> </span><br /> As the cod were all in spawning condition at the time of tagging and as they were released, within &frac12; nautical mile from where they were caught, on known spawning grounds, the first readings of depth and temperature will reflect the ambient environment characteristic for the spawning sites. The study of depth profiles during the following days and weeks after the tagging will therefore provide information that can be used to identify periods during which the cod is on a spawning site. <br /> <br /> Recordings made by those DST&rsquo;s that lasted more than a year, were often characterized by two distinct time periods of reduced vertical movements in shallow waters (fig.4). These time periods coincided with the recorded spawning time based on maturity stages (fig.2 ans 3). <br /> <br /> As an example, cod number 543 (fig. 4,&nbsp;5 and 6)&nbsp;was tagged on the main spawning grounds on April 24, 1996. At the end of the recovering period (see below) it had descended to 35-45 m depth, i.e. the depth range at which it stayed during the next two weeks (first period of reduced vertical movements). On May 12, it migrated into slightly deeper water and its vertical movements (fig.4-6)&nbsp;increased considerably indicating migratory activity. On May 25th it descended down below 100m depth and from there to more than 300m depth. Considerable vertical distance is from the site of release to where it could find such depths outside the continental shelf. <br /> <br /> This cod re-entered shallow water, for repeated spawning, on February 17, 1997 and stayed there until April 27, at which time it started to migrade back into deeper waters. A low activity period (reduced vertical migrations) was observed between March 31 and April 27 (fig.4 and 6). During this low activity period, the cod stayed at 35-45 m depth, similar to what was observed the year before. From April 27 to May 10 the cod descended to approximately 180 meters in three steps. During the rest of the recording time the cod displayed vertical migrations between 140 and more than 200 m. The last recordings made by the DST were on May 24, 1997. The cod was recaptured on September 3, off the South coast of Iceland in an area named Portland close to the edge of the continental shelf. Temperature profiles during the spawning season (fig.4-6)&nbsp;were typical for those recorded during surveys conducted on the main spawning grounds (Marteinsdottir et al., 1998), e.g. colder near the coast but gradually warming up during the spring effect. <br /> <br /> The two low activity periods in 1996 and 1997, i.e. from tagging to May 12 in 1996 and from March 31 &ndash; April 27 in 1997, are assumed to present the actual time period during which the fish holds station on the spawning ground. As no information exist on the behaviour of the fish prior to the tagging, the duration of stay during the first low activity period in 1996 remains unknown. Therefore, the duration of stay on the spawning grounds can only be obtained from tags that contain more than a whole year of recordings, or a complete period of low activity during a second year of recordings.<br /> <br /> Table 2. Range of depth in each spawning area. Mean and range of depth recorded by the DST&rsquo;s during stay on spawning grounds after post tagging recovery. Mean and range during the stay on the spawning grounds one year later. <br /> </p> <p></p> <table border="1" cellspacing="1" cellpadding="2" width="402"> <tbody> <tr> <td height="17" width="25%"> <p>Range of depth on spawning grounds</p> </td> <td height="17" width="36%"> <p>Mean and range of depth</p> <p>during the first pawning period I</p> </td> <td height="17" width="39%"> <p>Mean and range of depth during the second</p> <p>spawning period</p> </td> </tr> <tr> <td height="14" width="25%"> <p>40-60</p> </td> <td height="14" width="36%"> <p>50*</p> </td> <td height="14" width="39%"> <p>65(43-71)</p> </td> </tr> <tr> <td height="14" width="25%"> <p>40-60</p> </td> <td height="14" width="36%"> <p>55(53-58)</p> </td> <td height="14" width="39%">&nbsp;</td> </tr> <tr> <td height="14" width="25%"> <p>40-60</p> </td> <td height="14" width="36%"> <p>40(30-45)</p> </td> <td height="14" width="39%"> <p>41(29-60)</p> </td> </tr> <tr> <td height="14" width="25%"> <p>50-60</p> </td> <td height="14" width="36%"> <p>60(57-67)</p> </td> <td height="14" width="39%">&nbsp;</td> </tr> <tr> <td height="14" width="25%"> <p>50-60</p> </td> <td height="14" width="36%"> <p>59(43-65)</p> </td> <td height="14" width="39%">&nbsp;</td> </tr> <tr> <td height="14" width="25%"> <p>50-60</p> </td> <td height="14" width="36%"> <p>66*</p> </td> <td height="14" width="39%"> <p>60(53-66)</p> </td> </tr> <tr> <td height="14" width="25%"> <p>50-60</p> </td> <td height="14" width="36%"> <p>35(28-39)</p> </td> <td height="14" width="39%">&nbsp;</td> </tr> <tr> <td height="14" width="25%"> <p>50-60</p> </td> <td height="14" width="36%"> <p>70(60-73)</p> </td> <td height="14" width="39%">&nbsp;</td> </tr> <tr> <td height="14" width="25%"> <p>50-60</p> </td> <td height="14" width="36%"> <p>63(48-69)</p> </td> <td height="14" width="39%">&nbsp;</td> </tr> <tr> <td height="14" width="25%"> <p>50-70</p> </td> <td height="14" width="36%"> <p>62(57-75)</p> </td> <td height="14" width="39%">&nbsp;</td> </tr> <tr> <td height="14" width="25%"> <p>50-70</p> </td> <td height="14" width="36%"> <p>67(62-71)</p> </td> <td height="14" width="39%">&nbsp;</td> </tr> <tr> <td height="14" width="25%"> <p>50-70</p> </td> <td height="14" width="36%"> <p>68(65-69)</p> </td> <td height="14" width="39%"> <p>101(98-103)</p> </td> </tr> <tr> <td height="14" width="25%"> <p>50-70</p> </td> <td height="14" width="36%"> <p>67*</p> </td> <td height="14" width="39%">&nbsp;</td> </tr> <tr> <td height="14" width="25%"> <p>50-70</p> </td> <td height="14" width="36%"> <p>63(46-68)</p> </td> <td height="14" width="39%">&nbsp;</td> </tr> <tr> <td height="14" width="25%"> <p>50-70</p> </td> <td height="14" width="36%"> <p>60(48-67)</p> </td> <td height="14" width="39%">&nbsp;</td> </tr> <tr> <td height="14" width="25%"> <p>50-70</p> </td> <td height="14" width="36%"> <p>66*</p> </td> <td height="14" width="39%">&nbsp;</td> </tr> <tr> <td height="14" width="25%"> <p>50-70</p> </td> <td height="14" width="36%"> <p>62(49-69)</p> </td> <td height="14" width="39%">&nbsp;</td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>50-70</p> </td> <td height="14" valign="top" width="36%"> <p>60(46-72)</p> </td> <td height="14" valign="top" width="39%"> <p>79(69-86)</p> </td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>50-70</p> </td> <td height="14" valign="top" width="36%"> <p>77(56-78)</p> </td> <td height="14" valign="top" width="39%">&nbsp;</td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>50-70</p> </td> <td height="14" valign="top" width="36%"> <p>91(85-98)</p> </td> <td height="14" valign="top" width="39%">&nbsp;</td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>50-70</p> </td> <td height="14" valign="top" width="36%"> <p>61(57-70)</p> </td> <td height="14" valign="top" width="39%">&nbsp;</td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>46-70</p> </td> <td height="14" valign="top" width="36%"> <p>59(54-64)</p> </td> <td height="14" valign="top" width="39%">&nbsp;</td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>46-70</p> </td> <td height="14" valign="top" width="36%"> <p>59(53-63)</p> </td> <td height="14" valign="top" width="39%">&nbsp;</td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>46-70</p> </td> <td height="14" valign="top" width="36%"> <p>59(54-64)</p> </td> <td height="14" valign="top" width="39%">&nbsp;</td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>46-70</p> </td> <td height="14" valign="top" width="36%"> <p>58(48-65)</p> </td> <td height="14" valign="top" width="39%">&nbsp;</td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>90-110</p> </td> <td height="14" valign="top" width="36%"> <p>101(89-113)</p> </td> <td height="14" valign="top" width="39%">&nbsp;</td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>90-110</p> </td> <td height="14" valign="top" width="36%"> <p>103(95-109)</p> </td> <td height="14" valign="top" width="39%"> <p>100(81-111)</p> </td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>90-110</p> </td> <td height="14" valign="top" width="36%"> <p>142(137-145)</p> </td> <td height="14" valign="top" width="39%"> <p>148(132-159)</p> </td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>90-110</p> </td> <td height="14" valign="top" width="36%"> <p>109(100-111)</p> </td> <td height="14" valign="top" width="39%">&nbsp;</td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>90-110</p> </td> <td height="14" valign="top" width="36%"> <p>108(102-120)</p> </td> <td height="14" valign="top" width="39%"> <p>107(101-122)</p> </td> </tr> <tr> <td height="7" valign="top" width="25%"> <p>90-110</p> </td> <td height="7" valign="top" width="36%"> <p>109(108-110)</p> </td> <td height="7" valign="top" width="39%">&nbsp;</td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>90-110</p> </td> <td height="14" valign="top" width="36%"> <p>78(71-79)</p> </td> <td height="14" valign="top" width="39%"> <p>76(61-86)</p> </td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>100-120</p> </td> <td height="14" valign="top" width="36%"> <p>109(101-117)</p> </td> <td height="14" valign="top" width="39%">&nbsp;</td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>100-120</p> </td> <td height="14" valign="top" width="36%"> <p>107*</p> </td> <td height="14" valign="top" width="39%">&nbsp;</td> </tr> <tr> <td height="14" valign="top" width="25%"> <p>100-120</p> </td> <td height="14" valign="top" width="36%"> <p>110(100-114)</p> </td> <td height="14" valign="top" width="39%">&nbsp;</td> </tr> </tbody> </table> <p><br /> *Few recordings on depth as the fish left the spawning grounds soon after tagging<br /> <br /> Time of departure, arrival and duration of stay on the spawning ground based on DST&rsquo;s<br /> <br /> A similar behaviour as described for the DST in Figures 4-6, has been observed among the majority of the DST&rsquo;s recaptured. The depth of fish during the low activity period was typically variable but fell generally within the known depth range of the spawning ground where the fish were tagged (table 2). Time of departures from the spawning grounds was defined as the end of the low activity periods. The tagging procedure may affect the fish as well as the time spent on the spawning site. Therefore, dates of departure during the first (1996) and the second (1997) spawning season should be studied separately (Fig. 7). It should also be noted that some cod may have left the spawning grounds before the time of tagging. However, the range in departure time (26/4-23/5) is similar in both years. The distribution of points is very much the same in figure 7, for departures in 1996 and 1997 although the tagging was done 11 days earlier in the season in 1997 than in 1996.<br /> <br /> Figure 7 shows the relationship between the time of departure (days from 1st of January, 1996 or 1997) and the total-length of fish. The relationship is not significant (p &gt; 0.01), however it was observed that all of the largest cod (&gt; 98 cm long) left the spawning site before the May 10 (Julian day 130) while many of the smaller cod left the spawning site later. <br /> <br /> Until July 1998, only 11 of the DSTs recaptured from the study area provided long enough time series in order to show repeated spawning activity and the dates of arrival (Figure 8 and 9). Only 8 recaptures had information on sex, i.g. one female and 7 males. The relationship between size and time of arrival of males was weak, however significant (r2 = 0.15, p &gt; 0.01). It was observed that all of the larger males (&gt; 100 cm ) arrived before the end of March (Julian day 90) while two of the smaller males arrived later, i.e. in the middle of April. <br /> <br /> The length of duration of stay on the spawning ground did not appear to be size related. The shortest period of stay,14 days, was demonstrated by a 100 cm female. The longest duration, more than 33 days, was recorded by a DST recovered from a 90 cm long cod of an unknown sex. The average duration of stay among the males was 22.5 days ranging from 17 to 31 days.&nbsp;<br /> <br /> &nbsp;<br /> <br /> <span class="header1">Feeding activity during spawning migration</span><br /> <br /> During a 38 day period just after cod no. 543 re-enterd shallow water (Feb. 17- March 27, 1997, Figs. 4 and 6), considerable vertical movements were detected. This time period, characterised by shift vertical movements in relatively shallow water, most likely, represented a feeding period at which time the cod was probably chasing the capelin migrating along the coast along a route that often crosses the main spawning grounds of cod Southwest of Iceland. <br /> <br /> <span class="header1">Recovery period</span><br /> <br /> Many of the cod tagged during the experiments ascended up to the surface just after tagging. These cods stayed generally close to surface for a short time (Fig 5) after which they gradually descended and settled at depths that coincided with the range of depth on the spawning-ground. During this time period, the cod is recovering from the operation which has affected the pressure in the swimbladder. This behaviour has been observed in several tagging experiments using electronic tags, including transponding acoustic tags and DSTs with either internal or external attachments (Arnold and Walker, 1990,1992;God &amp; Michalsen 1997; Thorsteinsson, 1995).&nbsp;<br /> <br /> <span class="header1">Discussion</span><br /> <br /> Spawning-grounds of cod off the South- and West Coast of Iceland are well known from gill-net fishery data and gill-net surveys (Thorsteinsson, et al 1996, 1997 and 1998). These spawning-grounds of cod are generally associated with rocky elevations from the sea floor or steep banks that are inaccessible for bottom trawl. By using the obstacles on the seafloor for shelters the cod will spend less energy keeping station in an area of considerable tidal currents. (Gerstner, 1998; Dickson, 1989 a-b.). <br /> <br /> The present study indicates that temperature and especially depth profiles recorded by DST&rsquo;s can be used for identification of time periods spent by spawners on the spawning grounds. Arrival and departure on and from the spawning grounds appear to be clearly indicated by abrupt changes in depth towards a depth range found within the spawning grounds. Furthermore, the duration of stay on the spawning grounds is characterised by decreased movements reflected in less vertical migration. Additionally, on the second arrival, one year later, the same individual appears to reselect similar depth range as experienced before. <br /> <br /> Decrease in mobility is one of the characteristics of spawning aggregations of cod (Angelsen &amp; Olsen, 1987,Dickson 1989a-b). These authors showed that the catching efficiency of the gill-nets on the spawning grounds of cod, was higher during the time of arrival and departure and lower during the time period when most of the cod were settled on the spawning grounds. The most likely explanation for negative efficiency with increase in abundance is decrease in mobility (Borgstrm, 1991,). Thus inactivity or the holding station of cod on the spawning grounds during the spawning season can be related to the time of active reproductive behaviour. <br /> <br /> Until now, field studies on the size specific time and duration of spawning in cod as well as in other species (Hutchings and Myers, 1993; Marteinsdottir and Petursdottir, 1995; DeMartini and Fountain, 1981) have been limited to averages or frequencies of groups of spawners. The protracted period of spawning is achieved in part by repeated spawning of individual females and in part by asynchronisity of spawning among cod of different ages or sizes (Kjesbu, 1994; Hislop, et al , 1984; Hutchings and Myers, 1993). The data recorded by the DST&rsquo;s provides an opportunity to explore the duration of stay on the spawning grounds as well as the time of arrival and departure at the individual level. <br /> <br /> In the present study, the time and duration of spawning (based on maturity stages) appears to be more size specific among the females (especially in 1996) than among the males. Unfortunately, nearly all of the recoverd DST&rsquo;s were obtained from males. Only one female with a DST was recaptured during this time period. The results obtained for the males indicate that larger males arrive earlier and leave the spawning grounds earlier than smaller males. However, these results are based on weak relationships and only few observations (especially with respect to the time of arrival). Additionally, the duration of stay, among the males did not appear to be size related. Males in general stayed for 2-4 weeks on the spawning ground. Therfore, at least with regard to the males, the duration of the spawning period appeared to be achieved by protracted spawning of individuals as well as asynchronisity of spawning represented by variable time of arrival and departures of individual males. <br /> <br /> The present study is a first attempt to use DST&rsquo;s to explore duration of spawning periods. The results, although limited, are promising. In the future, an effort should be made to increase the number of females tagged, for example by developing a method that can be used to sex the fish at the time of tagging. It should also be noted that estimation of spawning time based on maturity stages is based on samples that are collected throughout the spawning season. In contrast, tagging is often performed over a short time period in the middle of the spawning season. Therefore, the sex ratio of spawning cod as well as the behaviour of those cod collected in the middle of the season may not represent those that were present in the beginning or in the end of the season. An effort should be made to tag cod, both prior to the spawning season as well as throughout the season. <br /> <br /> </p> <p><span class="header1">Literature Cited</span><br /> <br /> Angelsen K. &amp; Olsen S., 1987. Impact of fish density and effort level on caching efficiency of fishing gear. Fish. Res., 5 271-8.<br /> <br /> Arnold, G.P. and Greer Walker, M. 1992. Vertical movements of cod (Gadus morhua L.) in the open sea and the hydrostatic funcion of the swimbladder. - ICES J. mar. Sci., 49: 357-372.<br /> <br /> Arnold, G.P., Greer Walker, M., Emerson, L. S. and Holford, B.H. 1993. Movements of cod (Gadus morhua L.) in relation to the tidal streams in the southern North Sea. - ICES J. mar. Sci., 51: 207-232.<br /> <br /> Arnold, G.P., Lundgren, B. and God, O.R. 1994a. Electronic tags in fisheries research and management. Report on a workshop held at Fisheries Laboratory, Lowestoft, UK. 1417 November 1994. 61pp.<br /> <br /> Arnold, G.P., J.D. Metcalfe, B.H. Holford and A.A. Buckley, 1997. Availability and accessibility of demersal fish to survey gears: New observations of &quot;natural&quot; behaviour obtained with electronic data storage tags. ICES C.M. 1997/W:11.<br /> <br /> Baras, E., 1991. A Bibliography on Underwater Telemetry, 1956 - 1990. Canadian Technical Report of Fisheries and Aquatic Sciences 1819.<br /> <br /> Baras, E. and J.C. Philippart (editors) (1996). Underwater Biotelemetry, Proceedings of the First Conference and Workshop on Fish Telemetry in Europe, University of Liege, Belgium, vi+257pp.<br /> <br /> Borgstrm, R. 1992. Effect of population density on gillnet catchability in four allopatric populations of brown trout (Salmo trutta). Can. J.Fish. Aquat. Sci., 49, 1539-45.<br /> <br /> Chambers, R. C. &amp; Waiwood, K. G. 1996. Maternal and seasonal differences in egg sizes and spawning characteristics of captive Atlantic cod, Gadus morhua. Can. J. Fish. Aquat. Si. 53, 1986-2003.<br /> <br /> DeMartini, E. E. and R. K. Fountain, 1981. Ovarian cycling frequency and batch fecundity in the queenfish, Seriphus politus: Attributes representative of serial spawning fishes. Fishery Bulletin 79: 547-560<br /> <br /> Dickson, W. 1989a. Cod gillnet simulation model. Fish. Res., 7,149-74.<br /> <br /> Dickson, W. 1989a. Cod gillnet effectiveness related to local abundance, availability and fish movements. Fish. Res., 7,127-48.<br /> <br /> Engas, A. and Lokkeborg, S., 1994. Abundance Estimation using Bottom Gillnet and Longline &ndash; The Role of Fish Behaviour. In Marine Fish Behaviour in capture and abundance estimation, pp 134-165. Edited by Fern and S. Olsen. Fishing News Books.<br /> <br /> Gerstner C.L. 1998. Use of substratum ripples for flow refuges by Atlantic cod Gadus morhua. Environmental Biology of Fishes, 51: 455-460, 1998.<br /> <br /> God, O.R. &amp; Michalsen, K. 1997. The use of data storage tags to study cod natural behaviour and availability to abundance surveys in the Barents Sea. ICES C.M. 1997/W:18.<br /> <br /> Hislop, J. R.G. 1984. A comparison of the reproductive tactics and strategies of cod haddock, whiting and Norway pout in the North Sea In: G. W. Potts and R. J. Wootton (eds.), Fish Reproduction Strategies and Tactics. Academic Press, London. <br /> <br /> Hutchings, J. A. and R. A. Myers. 1993. Effect of age on the seasonality of maturation and spawning of Atlantic cod, Gadus morhua, in the Northwest Atlantic. Can. J. Fish. Aquat. Sci. 50: 2468-2474.<br /> <br /> Jonsson, E. 1982. A survey of spawning and reproduction fo the Icelandic cod. Rit. Fiskideildar 6: 45p.<br /> <br /> Jnsson, J. 1986. On the post spawning cod in Icelandic waters. ICES mimeo C.M. 1986/G:85.<br /> <br /> Marteinsdottir, G.&amp; Petursdottir, G. 1995. Spatial and temporal variation in reproduction of Icelandic cod at Selvogsbanki and nearby coastal areas. International Council for the Exploration of the Sea Committee Meeting 1995/G, 15.<br /> <br /> Marteinsdottir, G. og A. Steinarsson. 1998. Maternal influence on the size and viability of Iceland cod (Gadus morhua L.) eggs and larvae. J. Fish. Biol. 52 (6) : 1241-1258.<br /> <br /> Metcalfe, J. D., Fulcher, M. and T. J. Storeton-West 1992. Progress and developments in telemetry for monitoring the migratory behaviour of plaice in the North Sea. In WILDLIFE TELEMETRY Remote Monitoring and Tracking of Animals. Ed. Imants George Priede and Susan M. Swift, Department of Zoology, University of Aberdeen. Ellis Horwood.<br /> <br /> Metcalfe, J. D., G.P. Arnold and B. H. Holford 1994, ICES mini-symposium, CM 1994/Mini:11.T<br /> <br /> Mertz, G. and R. A. Myers. 1994. Match/mismatch predictions of spawning duration versus recruitment variability. Fish. Oceanogr. 3: 4, 236-245.<br /> <br /> Pedersen, B.H. &amp; Andersen N.G. 1985. A surgical method for implanting transmitters with sensors into the body cavity of cod (Gadus morhua). Dana, vol. 5, pp. 55-62.<br /> <br /> Thorsteinsson, V. 1995. Tagging experiments using conventional tags and electronic Data Storage Tags for the observations of migration, homing and habitat choice in the Icelandic spawning stock of cod. International Council for the Exploration of the Sea. ICES CM 1995/B:19 Ref.G<br /> <br /> Thorsteinsson, V., A. Gudmundsdottir, G. Marteinsdottir, G. Thorsteinsson and Palsson O.K. 1997. Gill-net survey to establish indices of abundance for the spawning stock of Icelandic cod in 1996. Survey report. HAFRANNSOKNASTOFNUN FJOLRIT NR.54.<br /> <br /> Thorsteinsson, V., A. Gudmundsdottir, and G. Marteinsdottir, 1998. Gill-net of spawning cod in Icelandic waters in 1997. Survey report. HAFRANNSOKNASTOFNUN FJOLRIT NR.66.<br /> <br /> Thorsteinsson, V. 1998. Vertical migration patterns of Atlantic cod (Gadus morhua) in Icelandic waters. Results from Electronic data storage tags (DSTs). 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