The North Sea cod stock assessment is based on indices of abundance-at-age from fishery-independent bottom trawl surveys. The age structure of the catch is estimated by sampling fish for otoliths collection in a length-stratified manner from trawl hauls. Since age determination of fish is costly and time consuming, only a fraction of fish is sampled for age from a larger sample of the length distribution and an age–length key (ALK) is then used to obtain the age distribution. In this study, we evaluate ALK estimators for calculating the indices of abundance-at-age, with and without the assumption of constant age–length structures over relatively large areas. We show that the ALK estimators give similar point estimates of abundance-at-age and yield similar performance with respect to precision. We also quantify the uncertainty of indices of abundance and examine the effect of reducing the number of fish sampled for age determination on precision. For various subsampling strategies of otoliths collection, we show that one fish per 5-cm-length group width per trawl haul is sufficient and the total number of fish subsampled for age from trawl surveys could be reduced by at least half (50%) without appreciable loss in precision.

中文翻译：

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北海国际底拖网调查中鳕鱼（Gadus morhua）年龄确定的抽样策略评估。

北海鳕鱼种群评估基于与渔业无关的海底拖网调查的成年指数。通过从拖网中以长度分层的方式对鱼进行耳石收集来估计渔获物的年龄结构。由于确定鱼的年龄是昂贵且费时的，因此从较大的长度分布样本中仅对一部分鱼进行年龄采样，然后使用年龄长度密钥（ALK）来获取年龄分布。在这项研究中，我们评估了ALK估计量，以计算在相对较大的区域上具有不变年龄结构的假设下的年龄人口丰度指数。我们表明，ALK估计量给出了类似的年龄点估计值，并且在精度方面产生了类似的性能。我们还量化了丰度指数的不确定性，并研究了减少用于年龄确定的采样鱼的数量对精度的影响。对于耳石收集的各种子采样策略，我们表明，每条拖网每5厘米长的群宽一条鱼就足够了，并且在不进行拖网调查的情况下，按照拖网调查年龄的子采样鱼总数至少可以减少一半（50％）精度明显下降。