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Abstract
Age structure data is essential for single species stock assessments but length-frequency data can provide complementary information. In south-western Australia, the majority of these data for exploited species are derived from line caught fish. However, baited remote underwater stereo-video systems (stereo-BRUVS) surveys have also been found to provide accurate length measurements. Given that line fishing tends to be biased towards larger fish, we predicted that, stereo-BRUVS would yield length-frequency data with a smaller mean length and skewed towards smaller fish than that collected by fisheries-independent line fishing. To assess the biases and selectivity of stereo-BRUVS and line fishing we compared the length-frequencies obtained for three commonly fished species, using a novel application of the Kernel Density Estimate (KDE) method and the established Kolmogorov–Smirnov (KS) test. The shape of the length-frequency distribution obtained for the labrid Choerodon rubescens by stereo-BRUVS and line fishing did not differ significantly, but, as predicted, the mean length estimated from stereo-BRUVS was 17% smaller. Contrary to our predictions, the mean length and shape of the length-frequency distribution for the epinephelid Epinephelides armatus did not differ significantly between line fishing and stereo-BRUVS. For the sparid Pagrus auratus, the length frequency distribution derived from the stereo-BRUVS method was bi-modal, while that from line fishing was uni-modal. However, the location of the first modal length class for P. auratus observed by each sampling method was similar. No differences were found between the results of the KS and KDE tests, however, KDE provided a data-driven method for approximating length-frequency data to a probability function and a useful way of describing and testing any differences between length-frequency samples. This study found the overall size selectivity of line fishing and stereo-BRUVS were unexpectedly similar.
Highlights
Fish length-frequency information can be used to gain an understanding of the biology and ecology of fish populations [1,2,3]
In south-western Australia, the majority of the age and length structure data used for the assessment and management of the exploited demersal fish species are derived from fisheriesdependent line caught samples [26,27].We investigated the relative biases and selectivities of stereo-BRUVS compared with fisheries-independent rod and line sampling for three exploited teleosts, baldchin groper Choerodon rubescens (Labridae, Gunther 1862) and breaksea cod Epinephelides armatus (Epinephelidae, Castelnau 1855), which are both endemic to the west coast of Australia, and the more widespread snapper Pagrus auratus (Sparidae, Bloch & Schneider 1801), which occurs across the southern half of Australia and northern New Zealand [28]
The length-frequency distribution of P. auratus sampled with stereo-BRUVS was bimodal, with a secondary mode in the histogram and Kernel Density Estimate (KDE) at a larger length (600 mm) that was not represented in the line fishing data
Summary
Fish length-frequency information can be used to gain an understanding of the biology and ecology of fish populations [1,2,3]. As ecosystem-based approaches to fisheries management are adopted around the globe (EBFM, [17,18]), it is becoming increasingly important to understand the predator-prey relationships of fished, by-catch and unfished species that could result in changes to assemblage composition Studies of these interactions would benefit from using methods that sample a representative range of species from different feeding guilds and trophic levels. Outside of protected areas stereo-BRUVS have been successfully used to correlate the abundance and biomass of exploited species across gradients in fishing pressure [24], and describe the consistent abundance distributions of endemic fish species across the old, climatically buffered seascape of south western Australia which provides a model system for biogeographic studies [25] It is likely the stereo-BRUVS method has very different relative biases and selectivities to traditional fisheries dependent and independent sampling methods [20]
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