Influence of fishers’ behaviour on the catchability of surface tuna schools in the Venezuelan purse-seiner fishery in the Caribbean Sea

Abstract

The influence of Venezuelan skippers' behaviour on the catchability of surface tuna schools was modelled using logistic regressions. Data obtained from observers onboard purse seiners indicated that fishers' fine-scale decisions, such as chasing and setting a school, were influenced by (i) the skipper's skill, (ii) the fishing equipment used (e.g., whether a bird radar was used or not), (iii) the features of the tuna school, and (iv) some environmental factors. The connections among the decisions related to the fishing process and these variables were synthesized in a causal network approach. Results clearly showed that schools (i) composed by fishes under 2 kg and (ii) where biomass was lower than 5 metric tons were disregarded by fishers. Because fishers used these thresholds to decide when to set the net, modifications in their behaviour over the years may introduce serious biases in the analysis of any catch rate and abundance relationship. An average daily activity record of a purse seiner shows the proportion of time spent in searching and in setting activities and highlights the impact that the use of new fishing equipment may have on fishing power.Résumé : L'impact du comportement des capitaines vénézuéliens sur la capturabilité des bancs de thons en surface a été modélisée à l'aide de régressions logistiques. Les données recueillies par des observateurs à bord des senneurs, permettent de relier les décisions des pêcheurs, telles que la poursuite du banc et le coup de filet, (i) au savoir-faire du capitaine, (ii) à l'utilisation de certains équipements technologiques (par ex., le radar à oiseaux), (iii) aux caractéristiques du banc de thons et (iv) à quelques facteurs environnementaux. Les relations entre les décisions impliquées dans le processus de pêche et ces facteurs ont été synthétisées à l'aide d'un réseau causal. Ce travail met clairement en évidence que les bancs (i) composés d'individus de moins de 2 kg et (ii) de biomasse inférieure à 5 tonnes étaient délaissés par les pêcheurs. L'existence de ces seuils dans la décision de tenter le coup de filet signifie que toutes modifications du comportement des pêcheurs au cours du temps pourra introduire de sérieux biais dans l'analyse des relations entre les rendements et l'abondance. Un résumé de l'activité journalière d'un senneur met en évidence le temps passé lors des phases de recherche du poisson et de sa capture et souligne le rôle des nouveaux équipements dans l'augmentation des puissances de pêche.Gaertner et al. 406

An investigation of human vs. technology-induced variation in catchability for a selection of European fishing fleets

“…Traditionally, linear models have been used to estimate fishing power while taking into account spatial and temporal heterogeneity of fish-populations and fishing activity (Gulland, 1964, Robson, 1966, Gavaris, 1980, Quirijns et al, 2008. When the residuals of such models indicate that there is evidence of more complex heterogeneity than could be explained by a simple spatial and temporal change in the data, it is common either to include interactions between these effects (Large, 1992, Maunder andPunt, 2004), or to consider the importance of environmental (Gaertner et al, 1999) or economic variables (Kirkley et al, 1995;Squires and Kirkley, 1999). Given the estimation of fishing power for each vessel of a fleet, identifying the most influential elements that affect a vessel's performance is an important step towards successful fisheries management.…”

Bycatch rates in fisheries largely driven by variation in individual vessel behaviour

“…There is ongoing debate about the key components of operator skill and its importance in different contexts, such as different gears or technical advancement of sheries [7][8][9][10] . Yet, numerous studies show consistent variation in target catch rates among anglers, skippers, or shing vessels that is not explained by environmental variables or economic inputs 7,[11][12][13] . This includes technically advanced and homogeneous eets where a skipper's skill would seemingly be less important 14 .…”

Fuel consumption and air emissions in one of the world’s largest commercial fisheries

“…At short time scales, fishing tactics depend on the technical skills and different sources of information available to the skipper, e.g., location of oceanographic features and acoustic estimates of tuna abundance around GPS-tracked FADs (Baidai et al, 2020;Gaertner et al, 1999). In addition, cooperative fishing is an essential component of purse seine fishing as FAD position information can be shared between vessels and some skippers work in groups (Lennert-Cody et al, 2020;Snouck-Hurgronje et al, 2018).…”