ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on July 27, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(8):1569-1572; doi:10.1093/icesjms/fsm115
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Seasonal variation in the trawl codend selectivity of picarel (Spicare smaris)
luEge University, Fisheries Faculty, 35100 Bornova, Izmir, Turkey
Correspondence to H. Özbilgin: tel: +90 532 706 19 77; fax: +90 232 374 74 50; e-mail: ozbilginh{at}yahoo.com or huseyin.ozbilgin{at}ege.edu.tr
Özbilgin, H., TosunoSeasonal selectivity of commercial 40 mm polyethylene codend was tested for picarel (Spicara smaris) in spring (4–18 April 2002), summer (10–25 July 2002), autumn (26 September–2 October 2002), and winter (22–23 January 2003) in the Aegean Sea. Data were collected using the covered codend technique and analysed using the logistic equation with the maximum likelihood method. Four sets of selection curves were analysed and compared using the model developed by Fryer (1991). Highest L50 is in autumn (13.82 cm; s.e. 0.62), when water temperature is highest and the fish are expected to be in their best condition after summer feeding. Lowest L50 is in spring (11.09 cm; s.e. 0.51), when water temperature is lowest and the fish are at their spawning stage. However, there is not sufficient evidence to say that the seasonal variation in the selectivity of 40 mm polyethylene codend for picarel is statistically significant (p > 0.05).
Keywords: Aegean Sea, picarel (Spicara smaris), seasonal variation, trawl selectivity
Received 26 July 2006; accepted 21 June 2007; advance access publication 27 July 2007.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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Catch per unit effort of conventional bottom trawls used in the Aegean Sea has decreased recently, and the mean body length of major species has become smaller (Tokaç et al., 1998). Possibly, poor gear selectivity is one reason for these changes, so a considerable number of selectivity studies have been carried out in Turkish waters. Although the codends tested in most of these studies were similar, their results for the same species were rather variable. One possible explanation for such variation is the effect of the experiments’ timing. In many studies, dates of individual hauls are not provided, and the data have been pooled over a variable period. There are not many studies in the literature investigating the effect of seasonal variation on codend selectivity. However, Özbilgin et al. (2006) reported that trawl codend selectivity for haddock (Melanogrammus aeglefinus) in the North Sea increased at the end of the summer feeding period, which coincided with the highest point of the annual water-temperature cycle. On the other hand, selectivity decreased in the post-spawning stage, which takes place in the low-temperature range in April. In addition, Özbilgin et al. (2005) reported a decreasing tendency in codend selectivity for annular sea bream (Diplodus annularis) in the Aegean Sea from summer to autumn, winter, and spring, but not at a statistically significant level.
Picarel (Spicare smaris) is a marketable fish in Turkey. However, although no minimum landing size is specified, smaller fish are usually discarded owing to low demand and prices. According to the State Institute of Statistics, in 2003, 1200 t of picarel were landed in Turkey (Anon., 2005).
In this study, we investigate seasonal changes in the selectivity of 40 mm nominal mesh size polyethylene (PE) codend used by commercial Turkish demersal trawlers in the eastern Aegean Sea. Selectivity of the same codend attached to a traditional gear was tested for picarel in four seasons under similar conditions, except for seasonal changes.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Codend selectivity data were collected in spring (4–18 April 2002), summer (10–25 July 2002), autumn (26 September–2 October 2002), and winter (22–23 January 2003). Fishing was done in Gülbahçe Bay, in the eastern Aegean Sea, during all trips. A traditional, 600 meshes around the mouth, commercial bottom trawl (see Tokaç et al., 1998, for drawing), with a 40 mm nominal mesh size PE netting codend, was used for fishing. The codend had 200 meshes around the circumference and a stretched length of 5 m. Its mesh size was measured as 42.4 mm (s.e. 0.26) using an ICES mesh gauge with 4 kgf (Tosuno
lu et al., 2003a). The same trawl was used in all operations, and all hauls were carried out aboard RV "Egesüf" (27 m, 500 h.p.). Tow duration was standardized at 45 min, and speed varied between 2.2 and 2.6 knots. Water depth in the fishing area varied between 25 and 30 m. Warp length was 150 m. All tows were carried out during daylight. A covered-codend method (Wileman et al., 1996; Tosunolu et al., 1997) was used to collect the selectivity data (Özbilgin and Tosunolu, 2003). At the end of each tow, we first removed the cover, then the codend catches, and the catches were sorted by species. Total lengths of the target species were measured to the nearest 0.5 cm. Selectivity parameters for individual hauls were estimated using MS Excel Solver (Tokai, 1997). The data were analysed using a logistic equation with the maximum-likelihood method (Wileman et al., 1996). Four sets of selection curves were estimated and compared using the model developed by Fryer (1991), which takes into account the between-haul variation in selectivity by allowing the selectivity curves to vary randomly around a mean selectivity curve according to a multivariate normal distribution. Codend catch size was used as an additional explanatory variable in the analysis, which was conducted by using EC-Model software (ConStat, Denmark).
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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A total of 38 hauls was conducted during four trips. However, individual haul data were obtained from 24 hauls: seven in spring, seven in summer, five in autumn, and five in winter. Total codend catch weights ranged between 27 and 61 kg in spring, 38 and 98 kg in summer, 58 and 192 kg in autumn, and 33 and 104 kg in winter. Codend catch size did not affect the selection parameters (p > 0.05) significantly.
Individual and mean selectivity ogives for each season are shown separately in Figure 1. The results demonstrate that the highest 50% retention lengths (L50) were in autumn, and the lowest in spring (Figure 2). L50 values of the mean curves were 11.09 cm (s.e. 0.51) in spring, 12.92 cm (s.e. 0.46) in summer, 13.82 cm (s.e. 0.62) in autumn, and 12.21 cm (s.e. 0.87) in winter. Selection ranges (SRs) were 2.60 (s.e. 0.15) cm in spring, 2.85 (s.e. 0.10) cm in summer, 3.08 (s.e. 0.14) cm in autumn, and 2.48 (s.e. 0.20) cm in winter. Seasonal differences in L50 and SR values (p > 0.05) were not statistically significant.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The study demonstrated that the estimates of L50 values follow a seasonal pattern. However, there was not sufficient evidence to say that the seasonal variation in selectivity of 40 mm PE codend for picarel is statistically significant (p > 0.05). This is mainly the result of the small number of hauls used each season. Although a total of 38 hauls was made during four trips, 14 had to be discarded because the number of picarel in the catch was insufficient to obtain individual haul-based selection parameters. A similar insignificant pattern in L50 values was also reported for annular sea bream in the Aegean Sea (Özbilgin et al., 2005).
On the other hand, a significant seasonal difference with the same pattern was observed for haddock in the North Sea (Özbilgin et al., 2006). In this study, the difference between autumn and spring L50 estimates was as high as 25% (Figure 2). Several factors play a role in selectivity: spawning period, condition of the fish, and water temperature, which changes seasonally.
A survey carried out in 2002 by the Ege University, Fisheries Faculty on the same fishing ground (unpublished data) reveals that the picarel's spawning period in Izmir Bay extends from March to July. In other words, during the spring trials of this experiment, the target species was at its spawning stage and, during the summer trials, the picarel had just completed spawning. Therefore, spawning exhaustion might have influenced the escape success of fish in these seasons.
Condition of the fish might affect selectivity (condition was not investigated in this study). Nevertheless, Özbilgin (1998) reported that the best selectivity for haddock was in September, when the fish were in their best condition after summer feeding. The worst selectivity was in April, when the fish were in their poorest condition at the post-spawning stage. Results of the present study are in line with those found for haddock by Özbilgin (1998); the highest L50 was in autumn and the lowest in spring.
It is well established that an increase in water temperature enhances swimming speed, and hence the escape performance of the fish (He, 1993; Videler, 1993; Özbilgin, 2002; Özbilgin and Wardle, 2002). Therefore, relatively warmer water temperature is expected to increase selectivity. Water temperature was not recorded during this study. Nevertheless, monitoring data collected by RV "K. Piri Reis" on the same fishing ground between 1994 and 2002 reveal that the bottom temperature varies between 13.4°C and 24°C. From these data, it can be estimated that approximate seasonal water temperatures during the experiments were 15°C in winter and spring, 19°C in summer, and 21°C in autumn. The increase in L50 from spring to summer and autumn can be attributed partially to the temperature increase.
Recently, two studies were completed on the selectivity for picarel in the eastern Aegean Sea (Tosunolu et al., 2003b; Tokaç et al., 2004). Both were carried out on a nearby fishing ground and followed methodology similar to that described here, but both analyses were based on pooled data instead of Fryer's (1991) between-haul variation method. Tokaç et al. (2004) conducted their trials between 4 February and 22 March 2002, and found an L50 of 12.3 cm (s.e. 0.10) and an SR of 2.8 cm (0.27). Tosunolu et al. (2003b) carried out the experiments between 9 August and 4 September 2002, and found an L50 of 13.5 cm (s.e. 0.16) and an SR of 1.89 cm (0.18). Both studies fit the pattern of seasonal changes in L50 and SR presented here.
In conclusion, the mean L50 of a commercial trawl codend for picarel is higher in summer and autumn than in winter and spring. Nevertheless, these differences are insignificant statistically. The pattern of change in estimates of seasonal mean L50 are considered to be relevant to changes in the water temperature, fish condition, and stage of spawning. A similar trend in L50 was reported for haddock (Özbilgin et al., 2006) and annular sea bream (Özbilgin et al., 2005), but further research is needed to determine whether it is applicable to other species in the trawl catch.
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Acknowledgements |
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The authors thank the captain and assistant scientific crew of the RV "Egesüf" for their help during the sea trials, the Deanship of the Ege University Fisheries Faculty for their financial support, E. Say
n for providing the temperature data, and T. Tokai for providing the program files used in estimating the selectivity parameters. |
References |
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