Similarities between Line Fishing and Baited Stereo-Video Estimations of Length-Frequency: Novel Application of Kernel Density Estimates

et al. (2012) Similarities between Line Fishing and Baited Stereo-Video Estimations of Length-Frequency: Novel Application of Kernel Density Estimates. PLoS ONE 7(11): e45973. doi:10.1371/journal.pone.0045973 Similarities between Line Fishing and Baited Stereo- Video Estimations of Length-Frequency: Novel Application of Kernel Density Estimates Timothy J. Langlois 0 Benjamin R. Fitzpatrick 0 David V. Fairclough 0 Corey B. Wakefield 0 S. 0 Alex Hesp 0 Dianne L. McLean 0 Euan S. Harvey 0 Jessica J. Meeuwig 0 David Mark Bailey, University of Glasgow, United Kingdom 0 1 The UWA Oceans Institute, The University of Western Australia , Perth, Western Australia , Australia , 2 School of Plant Biology, The University of Western Australia , Perth, Western Australia , Australia , 3 Centre for Marine Futures and School of Animal Biology, The University of Western Australia , Perth, Western Australia , Australia , 4 Western Australian Fisheries and Marine Research Laboratories, Department of Fisheries, Government of Western Australia , Perth, Western Australia , Australia , 5 Centre for Fish and Fisheries Research, School of Biological Sciences and Biotechnology, Murdoch University , Perth, Western Australia , Australia 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 fisheriesindependent line fishing. To assess the biases and selectivity of stereo-BRUVS and line fishing we compared the lengthfrequencies 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 lengthfrequency distribution for the epinephelid Epinephelides armatus did not differ significantly between line fishing and stereoBRUVS. For the sparid Pagrus auratus, the length frequency distribution derived from the stereo-BRUVS method was bimodal, 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. - Funding: The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. This research received funding from the Western Australian Marine Science Institute project to assist with the implementation of an ecosystem approach to the management of fisheries resources (TJL) and from the iVEC summer industry internship (BRF). Stereo-BRUVS data was sourced from Securing Western Australias Marine Futures a combined Australian and Western Australian Government Natural Heritage Trust (NHT) Strategic Project developed by South Coast Natural Resource Management, Monitoring of Reef Observation Areas at the Houtman-Abrolhos Islands a combined Australian and Western Australian Government NHT initiative through the Northern Agricultural Catchment Council, and Benchmarking of the Rottnest Island Marine Reserve developed by the Rottnest Island Authority. Fisheries Research and Development Corporation (FRDC 2000/137) funded line sampling for C. rubescens. Murdoch University funded line sampling for E. armatus. Pagrus auratus line sampling was funded by both Murdoch University and the Department of Fisheries. Competing Interests: The authors have declared that no competing interests exist. Fish length-frequency information can be used to gain an understanding of the biology and ecology of fish populations [1,2,3]. Biological parameters such as growth rate [4], maturity [5], functional sex for hermaphroditic species [6] and reproductive output [7] are all related to body length. This variable also correlates with catchability for a range of sampling gears such as traps [8], trawls [9] and longlines [10], and influences trophic interactions through size specific predator-prey relationships [11,12,13]. Length-frequency information can therefore provide additional information to compliment age data used for fish stock assessments [14,15] and studies of the ecological effects of fishing [16]. 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. Forward-facing baited remote underwater stereo-video (Fig. 1b, stereo-BRUVS) is a method that samples a wide range of fish species from a variety of trophic levels, including carnivorous, herbivorous and planktivorous fishes [19]. In addition to providing estimates of diversity and abundance [20], accurate length estimates of every fish sampled can be generated from stereoBRUVS systems [21,22]. These data have been shown to be useful for examining temporal trends in length-frequency data of an exploited coral reef species (Lethrinus miniatus) over several years inside and outside protected areas [23]. 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]. However, it is likely the stereo-BRUVS method has very different relative biases and selectivities to traditional fisheries dependent and independent sampling methods [20]. 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 (...truncated)