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Environmental influences on the daytime vertical distribution of Cape hakes and implications for demersal trawl estimates of hake abundance off the west coast of South Africa

机译:环境影响对Cape角的白天垂直分布以及对南非西海岸的hake丰度的底层拖网估计的影响

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摘要

The demersal fishery off the west coast of South Africa experiences decreased catches per unit effort of hake, Merluccius capensis and M. paradoxus, following the onset of strong south easterly winds. Research has demonstrated that, during daylight hours, Cape hakes migrate vertically in the water column in response to strong south easterly winds, decreasing their availability to the bottom trawl. Hydroacoustic, trawl and environmental data were collected off the West Coast during both calm and wind-swept periods in an attempt to understand the forces that initiate this behaviour, its spatial variability and the implications for demersal trawl estimates of abundance. Near-bottom currents appear to be the primary factor influencing the vertical distribution of the demersal fish community, of which hake constitutes a large proportion, during daylight hours. Correlation between wind and near-bottom currents suggest that the poleward component of the currents increase in velocity within eight hours following the onset of south easterly winds. The fish avoid boundary layers where currents change direction and speed dramatically, and seem to concentrate in waters with relatively stable current regimes. This result questions the assumption of CPUE-based assessment models that hake availability to the bottom trawl is constant or varies randomly. The incorporation of acoustic assessment techniques to demersal surveys has potential, but may be most valuable by supplementing swept-area estimates of abundance, since the sampling efficiency of these methods varies within the water column. The incorporation of wind indices and gear performance data to Cape hake assessment models have been identified as useful considerations for the future.
机译:南非西海岸的海域渔业在南风强东风袭击后,单位捕捞量的无须鳕,墨鱼和南美白对虾的捕捞量减少。研究表明,在白天,海角鳕鱼响应南风向东在垂直方向在水柱中迁移,从而降低了拖网对底拖网的利用。在平静和风吹扫的时期,在西海岸外收集了水声,拖网和环境数据,以了解引发这种行为的力,其空间变异性以及对深海拖网丰度估计的影响。在白天,近底流似乎是影响深海鱼类群落垂直分布的主要因素,其中无须鳕占很大比例。风与近底洋流之间的相关性表明,南极东风开始后八小时内,洋流的极向分量速度增加。鱼类避开了边界层,在该边界层中,电流的方向和速度急剧变化,并且似乎集中在电流状态相对稳定的水域中。这个结果质疑了基于CPUE的评估模型的假设,即对底拖网的可用性保持不变或随机变化。将声学评估技术结合到海底调查中具有潜力,但是通过补充扫掠区域的丰度估计值可能最有价值,因为这些方法的采样效率在水域内会有所不同。将风指数和齿轮性能数据纳入Cape Hake评估模型已被确定为未来的有用考虑因素。

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    Maree R C;

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  • 年度 2000
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  • 原文格式 PDF
  • 正文语种 English
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