SURVIVAL ESTIMATES OF BYCATCH INDIVIDUALS DISCARDED FROM BIVALVE DREDGES

The fate of released bycatch is an issue of great interest for fisheries research and management. Survival experiments were carried out to assess the survival capacity of animals damaged and discarded during clam dredging operations. Three common bycatch species, two fish (Trachinus vipera; Dicologlossa cuneata) and one crab (Polybius henslowii), were collected during the sorting of catches from a commercial dredging boat. An arbitrary score scale was used to quantify the type and extent of damage to the organisms. Onboard, damaged individuals were placed in tanks containing seawater which were subsequently transferred to the laboratory. Survival experiments were conducted during the subsequent 48h. D. cuneata exhibited the lowest mortality after 48h (54%), followed by P. henslowii (65%) and T. vipera (81%). Despite the magnitude of the percentage mortalities determined, the average number of individuals estimated to die during a 15 minutes tow (standard commercial fishing time) was relatively small: 1.2, 3.24 and 11 for D. cuneata, T. vipera and P. henslowii, respectively. Nevertheless, when these figures are extrapolated to cover all the dredging fleet the impact of this practice on the populations of the species studied can be significant, particulary for D. cuneata.

放生兼捕渔获物（bycatch）的存活结局，是渔业科研与管理领域备受关注的核心议题。本研究开展存活实验，以评估蛤类拖网捕捞作业中受损伤后被丢弃的生物的存活能力。研究选取了3种常见兼捕渔获物物种：2种鱼类（Trachinus vipera、Dicologlossa cuneata）与1种蟹类（Polybius henslowii），均采集自商业拖网渔船的渔获分拣过程中。研究采用任意设定的评分量表，对生物体所受损伤的类型与程度进行量化评估。在渔船甲板上，受损伤的个体被置于盛有海水的暂养箱中，随后转运至实验室。后续48小时内，研究人员开展了存活实验。48小时后，D. cuneata的死亡率最低（54%），其次为P. henslowii（65%）与T. vipera（81%）。尽管测得的死亡率绝对值较高，但按标准商业捕捞时长15分钟的拖网作业计算，预计死亡的个体数量相对较少：D. cuneata、T. vipera与P. henslowii的死亡个体数分别为1.2、3.24与11。然而，若将这些数据外推至全部拖网捕捞船队，则该作业方式对研究物种种群的影响可能相当显著，尤其针对D. cuneata而言。