The estimation of gear selectivity is a critical issue in fishery stock assessment and management.Several methods have been developed for estimating gillnet selectivity,but they all have their limitations,such as inap...The estimation of gear selectivity is a critical issue in fishery stock assessment and management.Several methods have been developed for estimating gillnet selectivity,but they all have their limitations,such as inappropriate objective function in data fitting,lack of unique estimates due to the difficulty in finding global minima in minimization,biased estimates due to outliers,and estimations of selectivity being influenced by the predetermined selectivity functions.In this study,we develop a new algorithm that can overcome the above-mentioned problems in estimating the gillnet selectivity.The proposed algorithms include minimizing the sum of squared vertical distances between two adjacent points and minimizing the weighted sum of squared vertical distances between two adjacent points in the presence of outliers.According to the estimated gillnet selectivity curve,the selectivity function can also be determined.This study suggests that the proposed algorithm is not sensitive to outliers in selectivity data and improves on the previous methods in estimating gillnet selectivity and relative population density of fish when a gillnet is used as a sampling tool.We suggest the proposed approach be used in estimating gillnet selectivity.展开更多
The selectivity of gillnets for Oreochromis niloticus in Amerti reservoir （9°63′ N, 37°23′ E） was determined from gillnets with four mesh sizes （60, 80, 100 and 120 mm）. Four selectivity models （a nor...The selectivity of gillnets for Oreochromis niloticus in Amerti reservoir （9°63′ N, 37°23′ E） was determined from gillnets with four mesh sizes （60, 80, 100 and 120 mm）. Four selectivity models （a normal model assuming fixed spread, a normal model assuming that spread is proportional to mesh size, a lognormal model and a gamma model） were fitted to the data by using the share each length＇s catch total （SELECT） method. A total of 657 specimens of Oreochromis niloticus were caught （12.0-35.5 cm total length, TD. The sizes at first sexual maturity were 21.5 cm TL and 18.9 cm TL, respectively, for male and female Oreochromis niloticus. The lognormal selectivity curve provided the best fit to the data according to model deviance estimates with optimum selectivity of 16.66, 22.26, 27.78 and 33.38 cm TL for the 60, 80, 100 and 120 mm mesh sizes, respectively.展开更多
In response to concerns over the mortality of discards and other negative environmental impacts of otter trawling for penaeids,the utility of bottom-set gillnets deployed by artisanal vessels to target Penaeus monodon...In response to concerns over the mortality of discards and other negative environmental impacts of otter trawling for penaeids,the utility of bottom-set gillnets deployed by artisanal vessels to target Penaeus monodon,Metapenaeus affinis and other retained species by-product was assessed in the Sea of Oman.Two gillnets(each comprising several 80-m panels for total lengths of up to 1.3 km)were diurnally fished at two locations(off Beris and Govater,eastern Iran)within established penaeid fishing grounds.During 30 days,the total catch was 2721 kg,comprising penaeids(981 kg),eight retained by-product species(598 kg and mostly Otolithes ruber,Cynoglossus bilinearus and Carangoides talamparoides)and 19 discarded species(1142 kg).The catch-per-unit-of-effort(CPUE,kg/gillnet-panel⋅hour-fished)of targeted penaeids and by-product varied significantly between areas(0.5-1.0 kg),but nevertheless reached economically viable levels.The CPUEs of total bycatch were similar at both sites(<0.5 kg)and no marine turtles were caught.The concomitant ratios of retained-catches-to-bycatch were considerably less than those for regional penaeid-trawl fisheries.Nevertheless,while gillnet selectivity was satisfactory for M.affinis,most of the P.monodon were smaller than their carapace length at maturity at both fished sites.The results support using gillnets to target penaeids and by-product species as an alternative to otter trawling in the Sea of Oman;albeit with ongoing assessments of modifications to maximise selectivity.展开更多
Gillnets are the primary fishing gear used to catch Atlantic cod(Gadus morhua)in coastal areas of Newfoundland and Labrador but are known to catch non-target species and produce lower quality fish than live-catch meth...Gillnets are the primary fishing gear used to catch Atlantic cod(Gadus morhua)in coastal areas of Newfoundland and Labrador but are known to catch non-target species and produce lower quality fish than live-catch methods.The purpose of this study was to compare the catch efficiency of collapsible cod pots against gillnets near the Gilbert Bay Marine Protected Area(MPA),Labrador,where a reduction in by-catch is needed.Results showed that one cod pot caught half as much Atlantic cod as a gillnet,and pots had a relatively stable catch rate throughout the sampling period,whereas gillnets caught relatively few fish later in the sampling season.Moreover,nearly all fish sampled with pots were caught alive undamaged,whereas most gillnet-caught fish experienced damage or mortality.As a conservation measure pot-fishing would enable fishing within close proximity to the Gilbert Bay MPA and the live-release of visually distinguishable Gilbert Bay cod,thereby mitigating against the negative effects of commercial fishing on this protected Atlantic cod population.展开更多
基金Supported by National Key Technology R&D Program of China(No.2006BAD09A05)
文摘The estimation of gear selectivity is a critical issue in fishery stock assessment and management.Several methods have been developed for estimating gillnet selectivity,but they all have their limitations,such as inappropriate objective function in data fitting,lack of unique estimates due to the difficulty in finding global minima in minimization,biased estimates due to outliers,and estimations of selectivity being influenced by the predetermined selectivity functions.In this study,we develop a new algorithm that can overcome the above-mentioned problems in estimating the gillnet selectivity.The proposed algorithms include minimizing the sum of squared vertical distances between two adjacent points and minimizing the weighted sum of squared vertical distances between two adjacent points in the presence of outliers.According to the estimated gillnet selectivity curve,the selectivity function can also be determined.This study suggests that the proposed algorithm is not sensitive to outliers in selectivity data and improves on the previous methods in estimating gillnet selectivity and relative population density of fish when a gillnet is used as a sampling tool.We suggest the proposed approach be used in estimating gillnet selectivity.
文摘The selectivity of gillnets for Oreochromis niloticus in Amerti reservoir （9°63′ N, 37°23′ E） was determined from gillnets with four mesh sizes （60, 80, 100 and 120 mm）. Four selectivity models （a normal model assuming fixed spread, a normal model assuming that spread is proportional to mesh size, a lognormal model and a gamma model） were fitted to the data by using the share each length＇s catch total （SELECT） method. A total of 657 specimens of Oreochromis niloticus were caught （12.0-35.5 cm total length, TD. The sizes at first sexual maturity were 21.5 cm TL and 18.9 cm TL, respectively, for male and female Oreochromis niloticus. The lognormal selectivity curve provided the best fit to the data according to model deviance estimates with optimum selectivity of 16.66, 22.26, 27.78 and 33.38 cm TL for the 60, 80, 100 and 120 mm mesh sizes, respectively.
文摘In response to concerns over the mortality of discards and other negative environmental impacts of otter trawling for penaeids,the utility of bottom-set gillnets deployed by artisanal vessels to target Penaeus monodon,Metapenaeus affinis and other retained species by-product was assessed in the Sea of Oman.Two gillnets(each comprising several 80-m panels for total lengths of up to 1.3 km)were diurnally fished at two locations(off Beris and Govater,eastern Iran)within established penaeid fishing grounds.During 30 days,the total catch was 2721 kg,comprising penaeids(981 kg),eight retained by-product species(598 kg and mostly Otolithes ruber,Cynoglossus bilinearus and Carangoides talamparoides)and 19 discarded species(1142 kg).The catch-per-unit-of-effort(CPUE,kg/gillnet-panel⋅hour-fished)of targeted penaeids and by-product varied significantly between areas(0.5-1.0 kg),but nevertheless reached economically viable levels.The CPUEs of total bycatch were similar at both sites(<0.5 kg)and no marine turtles were caught.The concomitant ratios of retained-catches-to-bycatch were considerably less than those for regional penaeid-trawl fisheries.Nevertheless,while gillnet selectivity was satisfactory for M.affinis,most of the P.monodon were smaller than their carapace length at maturity at both fished sites.The results support using gillnets to target penaeids and by-product species as an alternative to otter trawling in the Sea of Oman;albeit with ongoing assessments of modifications to maximise selectivity.
基金The study was funded by the Fisheries and Oceans Canada.
文摘Gillnets are the primary fishing gear used to catch Atlantic cod(Gadus morhua)in coastal areas of Newfoundland and Labrador but are known to catch non-target species and produce lower quality fish than live-catch methods.The purpose of this study was to compare the catch efficiency of collapsible cod pots against gillnets near the Gilbert Bay Marine Protected Area(MPA),Labrador,where a reduction in by-catch is needed.Results showed that one cod pot caught half as much Atlantic cod as a gillnet,and pots had a relatively stable catch rate throughout the sampling period,whereas gillnets caught relatively few fish later in the sampling season.Moreover,nearly all fish sampled with pots were caught alive undamaged,whereas most gillnet-caught fish experienced damage or mortality.As a conservation measure pot-fishing would enable fishing within close proximity to the Gilbert Bay MPA and the live-release of visually distinguishable Gilbert Bay cod,thereby mitigating against the negative effects of commercial fishing on this protected Atlantic cod population.
