Fleet dynamics of herring trawlers—change in gear size and implications for interpretation of catch per unit effort

Abstract

Catch per unit effort (CPUE) is widely used as an index of stock abundance. It is as widely acknowledged that CPUE can be a misleading index of abundance owing to a multitude of factors including fish behavior, fishing fleet interaction, and the increase in catchability over time caused by improvement in fishing technology. Based on information concerning the size of herring trawls manufactured in Finland since the early 1980s, an increase in fishing power of the fleet was postulated. Because we lacked direct information about the size of trawls aboard, we applied a model to estimate the changes over time. In the analysis, an analogy between fish and trawls was created by adopting the concepts and algorithms from fish stock assessment into assessment of the trawl "population", where both the total number of trawls and the size of individual trawls were being analyzed. The results indicate that the average gear size has nearly tripled in 20 years. Accepting the assumption that larger trawls are generally more effective than smaller ones, a substantial increase in fishing power has taken place. As a result, sequential population models calibrated with CPUE data will be severely biased as well.Résumé : Les captures par unité d'effort (CPUE) sont souvent utilisées comme indices de l'abondance des stocks, mais il est aussi généralement admis que les CPUE sont parfois des indices erronés à cause d'une multitude de facteurs, dont le comportement des poissons, l'interaction entre la flotte de pêche et les poissons et le raffinement de la technologie des pêches avec le temps. Compte tenu de la taille des chaluts à harengs construits en Finlande depuis le début des années 1980, nous avons postulé un accroissement du pouvoir de capture de la flotte. Bien que nous n'ayons aucun renseignement direct sur la taille des chaluts à bord, nous avons utilisé un modèle pour estimer les changements dans le temps. Dans cette analyse, nous avons fait une analogie entre les poissons et les chaluts en appliquant les concepts et les algorithmes utilisés pour l'évaluation des stocks de poissons à l'évaluation de la «population de chaluts» dans laquelle le nombre total de chaluts et leurs tailles respectives ont été analysés. Les résultats révèlent que la taille moyenne des engins de pêche a presque triplé en 20 ans. S'il est vrai que les chaluts plus grands sont généralement plus efficaces que les plus petits, un accroissement important du pouvoir de capture s'est donc produit. Il en résulte que les modèles démographiques séquentiels calibrés à partir des données de CPUE donnent lieu à des erreurs importantes.[Traduit par la Rédaction] Rahikainen and Kuikka 541

Effects of environmental variables on fish feeding ecology: implications for the performance of baited fishing gear and stock assessment

“…There are now a substantial number of investigations that show the sensitivity of stock assessment parameters to variation in active space (Sigler, 2000) or catchability coefficients (Hilborn & Walters, 1992; Arreguín‐Sánchez, 1996). Improvements are being made in models to incorporate more sources of variability (Cadrin, 2000; Maunder, 2001; McAllister & Kirchner, 2002; Rahikainen & Kuikka, 2002), particularly those based upon trawl surveys. As yet very few models adjust q to reflect changes in the physical or biological environment other than stock density, and further improvements in modeling are warranted.…”

Improving fishing effort descriptors: Modelling engine power and gear-size relations of five European trawl fleets

“…Number of meshes and mesh size in the fishing circle of a trawl is typically used to describe trawl size (e.g. Rahikainen and Kuikka, 2002;Eigaard and Munch-Petersen, 2010) but the size of the fishing circle can be transferred differently into geometrical components such as vertical or horizontal opening of the trawl mouth. Based on personal communications with net makers and fishermen as well as knowledge of how trawl geometry, ground gear and mesh sizes are best matched to the behaviour and size of target species, four conceptual trawl typologies were defined: 1) HV (herded volume) trawls are designed to catch shoaling pelagic or semi-demersal fish that are off the bottom such as herring (Clupea harengus), blue whiting (Micromesistius poutassou) and sand eel (Ammodytes marinus).…”

Unaccounted mortality in northern Baltic Sea herring fishery—magnitude and effects on estimates of stock dynamics

“…A structured interview study was conducted to obtain detailed information about herring trawlers' attributes related to fishing power and skipper's fishing strategies (Rahikainen and Kuikka, 2002). Both interview data and anecdotal information from fishers suggested systematic changes in the codend mesh sizes.…”

“…Effort, catch, and fishing mortality of herring by the Finnish trawling fleet strongly increased in the Bothnian Sea during 1990s (ICES, 2001b). Upsurge of effort took place not only as trawling hours, but also the area swept increased due to marked enlargement in the trawl size (Rahikainen and Kuikka, 2002). It is most likely that the fraction of the herring stock encountering trawls has increased accordingly.…”