Deep-sea sharks as by-catch of an experimental fishing survey for black scabbardfishes (Aphanopus spp.) off the Canary Islands (NE Atlantic)

Abstract

The deep-sea sharks associated as by-catch of the Madeiran midwater drifting longline fishery for scabbardfishes (Aphanopus spp.) were investigated by means of an experimental survey at 800-1200 m depth within the Canary Islands Exclusive Economic Zone, whose fishing grounds have been exploited during the past 15 years. Nine species of chondrichthyans were identified, belonging to five families: Pseudotriakidae, Centrophoridae, Etmopteridae, Somniosidae and Chimaeridae. Data on length, weight and sex ratio for the 436 chondrichthyan individuals caught in the March 2009 survey are given. Several voucher specimens of each species caught were deposited in the collections of the Natural History Museum of Funchal.

INTRODUCTION

The deep-sea fishery of the bathypelagic black scabbardfishes, Aphanopus carbo and A. intermedius, has a long tradition off the Portuguese mainland and around the Madeira archipelago, representing one of the world’s longest standing exploitations of meso- and bathypelagic fishes (Delgado et al. 2013, 2018). Off Madeira this fishery is performed with midwater drifting longlines set between 800 and 1200 m depth over the continental slope and nearby seamounts. This is one of the oldest known fisheries in the world targeting a deep-water resource (Leite 1988).

A preliminary study on the by-catch and incidental catch of a potential black scabbardfish (Aphanopus spp.) fishery off the Canary Islands, based on a few experimental surveys on board research vessels, was published by Pajuelo et al. (2010).

Ten years ago, Portugal and Spain signed an agreement to study the impact of the black scabbardfish fishery off the Canary Islands, using commercial fishery vessels from Madeira in an experimental survey. The main objectives were i) to study deep-sea sharks as a by-catch of this Madeiran fishery off the Canary Islands, and ii) to compare this by-catch with the by-catch off Madeira and the Portuguese mainland with the same fishing system. The results of this survey are now presented on this short paper.

MATERIALS AND METHODS

In March 2009 an experimental fishing survey was carried out off the Canary Islands between 800 and 1200 m depth using black scabbardfish midwater drifting longlines of the Madeiran type. This fishing gear has 5000 hooks (type 6) (Martins and Ferreira 1995) and is usually set well above the bottom in the water column between 700 and 1300 m depth, without contacting the seafloor (Delgado et al. 2018). Fishing operations were carried out by two Madeiran professional vessels, F/V Pico Dourado and F/V Pico Alto, with scientific observers on board. In accordance with Canary Islands legislation restrictions (no more than 500 hooks per longline are permitted), both vessels used two separate longlines, one much longer than the other: a professional one with around 5000 hooks and an experimental one with around 500 hooks. The neon flying squid Ommastrephes bartramii was used as bait in all longlines. Twenty hauls were carried out off the islands of Tenerife, La Gomera, La Palma, Fuerteventura and El Hierro (Fig. 1). The sampled areas were not randomly selected, but based on the captains’ experience from the fishery grounds, as well as on the target species’ vertical distribution in the area (Pajuelo et al. 2008).

[Figure omitted. Please see PDF.]

Shark individuals caught were identified following Compagno et al. (2005) and then counted and weighed by species on board. Catches of chondrichthyans and CPUE values by fishing operation (location) were calculated, as well as an average CPUE per island. CPUE values were expressed in kg (total weight) per 1000 valid hooks. For each species caught, all individuals or well-representative subsamples were selected and preserved on board and then measured (total length TL, in mm), weighed (total weight TW, in g), and sexed at the laboratory.

Several voucher specimens of each species caught were deposited in the collections of the Natural History Museum of Funchal (MMF).

RESULTS

A total of 436 individuals of 9 mesopelagic species of Chondrichthyes belonging to 5 families (Pseudotriakidae, Centrophoridae, Etmopteridae, Somniosidae and Chimaeridae) were captured. The leafscale gulper shark (Centrophorus squamosus) (Centrophoridae), represented by 170 individuals, was the most abundant species. The velvet dogfish (Zameus squamulosus) (n=129) and the roughskin dogfish (Centroscymnus owstonii) (n=121) (both Somniosidae) were well-represented in the catches. The capture of the other five chondrichthyans could be considered as incidental (Table 1).

Table 1. – Chondrichthyan species caught, with number of individuals, descriptive statistics for size and weight, and sex ratio (males:females). Size as total length (TL); weight as total weight (TW).

[Table omitted. Please see PDF.]

Apart from the list of chondrichthyan species caught, Table 1 also indicates the number of individuals captured, the descriptive statistics for size and weight, and the sex ratio (males:females) for each species.

Table 2 show catches of chondrichthyans and CPUE values per location. Mean CPUE per island surveyed varied from 0.00 off Lanzarote (no shark was caught) to 182.96 kg/1000 hooks off Fuerteventura, with low CPUE values for the remaining sampled islands.

Table 2. – Catches of chondrichthyans and CPUE values per fishing operation (location), with an average CPUE per island. CPUE expressed in kg (total weight) per 1000 hooks.

[Table omitted. Please see PDF.]

DISCUSSION

With regard to the European Red List of Marine Fishes (Nieto et al. 2015), it is important to highlight that the leafscale gulper shark (Centrophorus squamosus), the most captured species in the survey, has been classified as an endangered species. Previously, this shark species was considered as highly vulnerable to depletion and consequently assessed as endangered in the northeast Atlantic (White 2003). Also, this species was included in the OSPAR List of threatened and/or declining species and habitats (Gibson et al. 2008). Moreover, the gulper shark (Centrophorus granulosus) has been recently catalogued as a critically endangered species (Nieto et al. 2015).

When compared with that of the by-catch identified from the black scabbardfish fishery off Madeira (Bordalo-Machado et al. 2009), chondrichthyan species composition was quite similar to the present results for the Canary Islands waters, except for the absence of the Portuguese dogfish Centroscymnus coelolepis (Somniosidae) and the birdbeak dogfish Deania calcea (Centrophoridae) off the Canaries. However, given that the present survey was carried out only in March, any type of seasonal distribution pattern in those species would have been missed.

In the eastern Atlantic, landings of the leafscale gulper shark, C. squamosus, the most captured species within the by-catch, over the past decade peaked in 2003 (3042 t) and declined to 243 t in 2009 (Ebert and Stehmann 2013). Based on FAO catch data, in Portugal average landings of this species were 893 t per year from 2000 to 2009. In contrast, short time series of CPUE available for the western coast of Portugal seem to indicate that these landings remain stable (Gibson et al. 2008).

According to the present results and concerns, further studies should be conducted in order to evaluate the real impact of this fishery on the northeastern Atlantic population(s) of the leafscale gulper shark, and should also address technological developments related to the fishing gear tending to minimize its catches.

In order to fully understand the trends of all species caught as by-catch in this fishery, it is important to monitor continuously to obtain representative time series.

ACKNOWLEDGEMENTS

The authors are indebted to the masters and crews of the F/V Pico Dourado and F/V Pico Alto for all their work done at sea. Thanks are also due to the technicians of the Estação de Biologia Marinha do Funchal, Direcção de Serviços de Investigação das Pescas, Instituto Español de Oceanografía and Instituto Canario de Ciencias Marinas, who collaborated at sea and in the laboratory. Many thanks to the reviewers for their comments, which greatly improved our manuscript.

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