The Information Value of Full-Retention Policies

Discard rates in marine fisheries have been estimated at more than 80% for some individual fisheries, with an average global discard rate of 8%. Discarding of catch can be problematic for three main reasons: (1) in the absence of accurate and precise discards estimates, unreliable catch data distort estimation of the appropriate quotas; (2) bycatch imposes a cost on the resource as survival rates are generally low; and (3) bycatch-induced mortality of charismatic species presents a loss of non-use values. Policy instruments to limit discarding vary from taxes on bycatch, to subsidies for selective fishing gear, to outright bans on discarding, also called full-retention policies. The 2013 reform of the European Union's Common Fisheries Policy features the introduction of such an obligation to land all catches "of species which are subject to catch limits." In this paper we aim to estimate the economic effects of a full retention policy in a mixed fishery. More specifically, we estimate the economic value of the data distortion caused by discarding of fish, and how these economic losses are ameliorated by a ban on discards. We present a bioeconomic model capable of investigating these questions in a mixed fishery for a variety of stock assessment methods, harvest control rules, and discarding policies. We apply the model to the North Sea demersal fishery, focusing on the two main commercial species plaice (pleuronectes platessa) and sole (solea solea). We find that the information value of discard bans depends largely on the prevailing harvest control rule.Keywords: Poor Data and Uncertainty, Fisheries Economics, Modeling and Economic Theor

Quantifying habitat preference of bottom trawling gear

Continental shelves around the world are subject to intensive bottom trawling. Demersal fish assemblages inhabiting these shelves account for one-fourth of landed wild marine species. Increasing spatial claims for nature protection and wind farm energy suppresses, however, the area available to fisheries. In this marine spatial planning discussion, it is essential to understand what defines suitable fishing grounds for bottom trawlers. We developed a statistical methodology to study the habitat preference of a fishery, accounting for spatial correlation naturally present in fisheries data using high-resolution location data of fishing vessels and environmental variables. We focused on two types of beam trawls to target sole using mechanical or electrical stimulation. Although results indicated only subtle differences in habitat preference between the two gear types, a clear difference in spatial distribution of the two gears was predicted. We argue that this change is driven by both changes in habitat preference as well as a change in target species distribution. We discuss modelling of fisheries' habitat preference in light of marine spatial planning and as support in benthic impact assessments.</p

A preliminary habitat suitability model for oceanic whitetip shark in the western Indian Ocean

Understanding the temporal, spatial and environmental factors influencing species distributions is essential to minimize the interactions of vulnerable species with fisheries and can be used to identify areas of high bycatch rates and their environmental conditions. Classified as critically endangered by the International Union for the Conservation of Nature, the oceanic whitetip shark (Carcharhinus longimanus) is the second main shark species incidentally caught by the tropical tuna purse seine fishery in the western Indian Ocean. In this study, we used the European Union purse seine fishery observer data (2010-2020) and generalized additive models to develop a habitat suitability model for juvenile oceanic whitetip shark in the western Indian Ocean. Sea surface temperature was the main environmental driver suggesting a higher probability of occurrence of this shark with decreasing temperatures. The type of fishing operation also was an important predictor explaining its occurrence, suggesting a higher probability of incidentally catching this species when using fish aggregating devices as set type. Moreover, predictive maps of habitat suitability suggested the area offshore of Kenya and Somalia are an important hotspot with higher probabilities of incidentally catching this species during the summer monsoon (June to September) when upwelling takes place. The habitat suitability models developed here could be used to inform the design and testing of potential time-area closures in the Kenya-Somalia basin with the objective of minimizing the bycatch of this critically endangered species with the least possible impact on fishing operations and fishery yields of target tuna

Effort allocation of the Dutch beam trawl fleet

Dit proefschift gaat in op de vraag hoe vissers hun activiteiten verdelen in ruimte en tijd. Het begrijpen van de mechanismen die de dynamiek van vissersvloten bepaalt is van belang om te begrijpen hoe vissers reageren op maatregelen in het kader van visserijbeheer en veranderingen in de omgevingsvariabelen. Deze kennis kan gebruikt worden door visserijbeheerders om effectieve beheersmaatregelen te ontwerpen. Het onderzoek richt zich op demersale visserijen die een aantal soorten tegelijkertijd exploiteren. De resulltaten in het proefschrift laten zien dat de visserij reageert op seizoensveranderingen in de ruimtelijke verdeling van hun doelsoorten schol en tong. Gebiedsspecialisatie en interferentiecompetitie tussen schepen spelen een rol in de totstandkoming van de ruirntelijlce verdeling van de vloot, binnen de beperkingen die er opgelegd worden door het visserijbeheer. Er is een 'dynamic state variable model' ontworpen om het effect van beperkingen als gevolg van visserijbeheer op de ruirntelijke en temporele verdeling van visserijinspanning en discarding te bestuderen. Dit model evalueert de trade-offs en beperkingen waar individuele vissers mee geconfronteerd worden in hun visserij. Beheersrnaatregelen be'invloeden de trade-offs en beperkingen die het individuele gedrag van vissers vormgeven, wat resulteert in veranderingen in de riuimtelijke verdeling van vissersvloten en andere aspecten van vloot dynamica. In het geval van de boomkorvloot laat het onderzoek zien dat de visserij-inspanning als gevolg van de verlaging van het scholquotum zuidwaarts is verplaartst. Dit heeft tot gevolg gehad dat er een toename is geweest van de relatitieve vangbaarheid voor tong en jonge schol, en een drijfveer voor vissers om marktwaardige vis te discarden.