Bycatch estimation of Rajiformes in multispecies and multigear fisheries

Highlights

• Species misidentification and lack of fishing gear data in official statistics.

• Statistical stepwise procedure to estimate landed weight by species and effort.

• Relevance of dedicated sampling programs for by-catch species.

• Collaboration with fishermen as a source of relevant fisheries knowledge.

Abstract

To assess the exploitation status of marine resources and to formulate scientific advice for fisheries management, reliable fishery data are required. When dealing with data-poor stocks, like skates and rays (Rajiformes) caught by multispecies and multigear fisheries in Portugal, strategies need to be defined to surpass data limitations, like species misidentification and the lack of fishing gear information in landings data. This paper presents a stepwise statistical procedure, involving factor analysis for mixed data and flexible discriminant analysis, to estimate the total number of trips and the correspondent total landed weight by species. Interviews and self-sampling that took place under a pilot study (2011 – 2013) focused on skate and ray fisheries showed that collaborative research is a low cost and valuable tool to improve our knowledge on fisheries and their catches and may contribute to increase the accuracy. Given the persistence of species misidentification errors in the official statistics, the procedure here presented is currently used to report the Portuguese annual estimates of species-specific landed weight. This procedure may be considered to be applied to data deficient fishery bycatch resources for which dedicated sampling programs are not priority.

Introduction

Rajiformes (skates and rays) are important marine resources exploited by commercial European fleets. The scientific advice for the most important species of Rajiformes is provided by the International Council for the Exploration of the Sea (ICES) on a species-specific basis. The European Union (EU), however, applies a generic Total Allowable Catch (TAC) for all Rajiformes combined. This management rule is of ecological concern as not all the species have the same resilience to fishing. A generic TAC may not provide the appropriate protection for stocks whose status determine the adoption of specific management measures (STECF, 2015, 2017).

Given the different resilience between rajiform species to fishing, EU set regulations that oblige member states to report commercial catches of Rajiformes by species (EC, 2009, 2013). The list of species for this mandatory requirement varies between the different ICES ecoregions and advisory areas. In 2009 and for the Biscay and Iberian waters ecoregion the list included the separate report of cuckoo ray (Leucoraja naevus) and thornback ray (Raja clavata) landings (EC, 2009). In 2013 this list was extended to blonde ray (Raja brachyura) (EC, 2013).

In Portugal mainland, historically, the official landings of rajiform species were made under a generic category (skates and rays nei) since the middle of the last century (Figueiredo et al., 2007). During the 90’s and given a different commercial value assigned to L. naevus in certain Portuguese landing ports, the landings of this species were firstly separated from the remaining Rajiform species and landings assigned to the commercial designation “raia de S. Pedro” (“sandy ray”, Leucoraja circularis). In 2000, the commercial designation “raia lenga” (“thornback ray”) was introduced in the official landings statistics but given the similarity on the price per kg between different species, landings of “thornback ray” were likely to include other rajiform species. In 2005, three new commercial designations were included in the official statistics: “raia pontuada” (“blonde ray”), “raia manchada” (“spotted ray”, Raja montagui) and “raia de dois olhos/raia de São Pedro” (“cuckoo ray”). Despite this intention to separate rajiform' landings at species level, misidentification problems persisted (ICES, 2018).

A preliminary study on Rajiformes conducted by the Portuguese Institute for Sea and Atmosphere (IPMA) in 2001 at the landing ports of Matosinhos (north region) and Peniche (center region) observed that the generic category included eight different species: L. naevus, R. brachyura, R. clavata, Raja microocellata, Raja miraletus, Raja montagui, Raja undulata and Rostroraja alba (Bordalo-Machado et al., 2004).

In 2017, the Portuguese Directorate General for Natural Resources (DGRM) reported Rajiformes represented around 48% of the total annual official landed weight of elasmobranchs in Portuguese continental waters. That group of species are caught as bycatch from multispecies and multigear fisheries, mainly from the Portuguese polyvalent segment (around 83% of the total landed weight of Rajiformes). The Portuguese polyvalent segment includes vessels with overall length (LOA) ranging from 5 to 27 m and that can own several fishing licenses that enable them to use one or more types of fishing gear by trip (including for example gillnets, trammel nets, traps and longline). The fisheries of the polyvalent segment capture a high diversity of species at different fishing grounds. Those features are reflected by the variation of the target species and/or fishing ground(s) by each vessel (Duarte et al., 2009). Each polyvalent vessel's fishing dynamics and clustering of fishing patterns within the fleet are difficult to determine due to regional differences and the unreported fishing gear used in the fishing trips. And although vessels with LOA larger than 12 m are obliged to fill logbooks with the fishing gear used, not all the fishing trips are registered in the DGRM database, and for those for which there is information there are inconsistencies on the data recorded, (e.g. fishing gear, species caught).

To improve the knowledge on the Portuguese fishery data on Rajiformes, IPMA started in 2011 a three-year Pilot Study. The target population of this Pilot Study was the fisheries belonging to the polyvalent fleet with landings of Rajiformes in mainland Portugal. The main objective of the present work was to estimate the species-specific landed weight and the corresponding number of fishing trips derived from the Portuguese polyvalent fleet. For that, a statistical stepwise procedure based on official data, scientific sampling information and fishermen knowledge was developed.