Effect Of Escape Vent On Selectivity Of Crab Pots

The crab pot fishery in East China Sea originated in early 1980 s. Since the early 1990 s, crab pots have been developing rapidly for its simple structure, easy operation, low energy, small demand for labor and strong adaptability for fishermen, etc. As the expansion of fishing ground and the increase of fishing effort, it have put a heavy pressure on the crab sources and fishery production has fluctuated successively. Therefore, the study on the influence of escaped vent of on selectivity of crab pots is designed to enact reasonable management measures of escape vent, keep the stability of offshore crab resources and provide scientific basis for the sustainable development of fishery.From October 2012 to December 2013, the project team has carried out selectivity experiments of crab pots in a period of four voyages for a total of 25 nets of sea trials by a commercial fishing vessel “Zheshengyu 09101”. In the experiments, the fishery biology characteristics of the main catch species of crab pots, including Portunus trituberculatus and P. sanguinolentus were measured and analyzed. Combined with SELECT analysis model and mixed effect model, location of escape vent on size selectivity of crab pots has been analyzed by installing rectangle escape vents on one side(at the top or bottom) and on both sides(at the top and bottom) of crab pots by comparison experiment. And different heights of escape vent(2.0cm、2.5cm、2.8cm、3.0cm、3.3cm and 3.5cm respectively) on size selectivity of rectangle escape vents for swimming crabs were compared. Moreover, in order to better reflecting the selectivity performance of crab pots, control stownets were compared to analyze the relative selectivity based on SELECT analysis model.The results showed that size of catch group of autumn 2012 and winter 2013 was larger than that of spring and autumn of 2013. The size of male individuals was slightly larger than the female individuals. The relationships between carapace width(CW) and body weight(W) of P trituberculatus and P. sanguinolentus were W=5.077×10-4CW2.533 and W=1.388×10-4CW2.797 respectively. The gonad development of female and male P trituberculatus caught in autumn and winter distributed in Ⅰ-Ⅲand Ⅱ-Ⅴ phase respectively. The condition factor of P. trituberculatus is 5.65±0.94g/mm3, and significantly negative correlated to carapace width. As for P. sanguinolentus, the gonad development of female and male individuals distributed in Ⅱ-Ⅲand Ⅲ-Ⅴ phase respectively and the condition factor is 5.78±0.75 g/mm3.The results also demonstrated that the selectivity indexes for 50% selectivity carapace width(CW50) and selective range of carapace width(SR) of crab pot with two escape vents on both sides are a little bigger that that with only one escape vent. But the 95% confidence intervals are in good agreement which means that the discrepancy is small. The 50% retention carapace width(CW50) increased with the increase of the escape vent height for target species of crab pots with escape vents. But the relationship between the selectivity range and escape vent height is not clear. From the linear relationship between carapace width and carapace height, the 50% retention carapace height(CH50) and escape vent height(VH) indicated a significant linear relationship. They are CH50=0.656×VH+9.30 for Portunus trituberculatus and CH50=0.636×VH+8.71 for P. sanguinolentus. The smaller slope means the small individuals are more willing to escape from the escape vent than big crabs.The individual sizes of stownets catch were smaller than those of control crab pots in contrast with the catch groups in stownets. Compared with the catch frequency of control pot, the encounter selectivity of experimental crab pots was analyzed. The selectivity demonstrated that the 50% retention carapace width(CW50) and fishing power increased while the selection range(SR) kept unchanged with the incensement of the width of escape gap. Compared with the catch frequency of stownet, the relative selectivity of crab pots for swimming crab was estimated. The relative selectivity of the crab pots was treated as the catchability based on the fishing characteristics of pots and stownet. According to the catchability, we deduced that the big crabs were more vulnerable to contact and then enter the crabs than the small crabs and the 50% probability contact carapace width was 98.7mm. As for the experimental crab pots with 30 mm escape gap width, there was little difference in selectivity indexes between the relative and encounter selectivity. The CW50 and SR of relative selectivity are 118.4mm and 10.5mm while those of encounter selectivity are 117.4 mm and 9.5mm, respectively. As for experimental crab pots with 25 mm escape gap width, there was no difference in CW50 between the relative and encounter selectivity. However, the SR of relative selectivity(5.6mm) was significantly less than encounter selectivity(11.3mm). Therefore, there would be some bias in population assessment if only the encounter selectivity information was taken into account while the catchability was omitted. Compared to the results of early studies, the abnormal size frequencies, decrease of exponents of relationship between CW and W and condition factor suggested the possible trends of intensification of fluctuations in crab stock and population miniaturization, growth acceleration and development slowing down of crab individuals. According to the results, the location of escape vent on performance of crab pots has no significant influence. So the author considered that it is practicable to install only one escape vent on the crab pot in the present East China Sea crab pot fishery. And rectangular escape gaps with 30 mm height was recommended when the biological information of swimming crabs and commercial demand of fisheries were taken into account.