Biology, Taxonomy and Distribution of south-east Pacific Cartilaginous Fishes

Abstract

Cartilaginous fishes (formerly Class Chondrichthyes) belong to an ancient, highly conserved taxonomic group. Despite the evolutionary success of the group, of which there are over 1,000 extant species, many are now threatened with extinction, regionally or globally, primarily as a result of human activities. The main reason for the susceptibility of these species is the K-selected life-history traits that characterise many species of cartilaginous fishes such as, slow growth, relatively late maturity, low fecundity and a low rate of potential population increase. Cartilaginous fishes are generally predators that occupy the upper levels of marine food webs. They are naturally rare compared with other fishes, but have a vital role in the maintenance of marine ecosystem balance. Their biological characters make them highly susceptible to population depletion as a result of anthropogenic activity, including targeted fisheries, bycatch and habitat loss or modification. The life history strategy of cartilaginous fish species suggests the need for conservative management measurements that ensure sustainability with compatible fishery activities. Cartilaginous fishes constitute an important component of the marine ecosystem in the south-east Pacific Ocean. Over 93 species of sharks, skates, rays and chimaeras inhabit Chilean waters and at least half of them are subjected to intense fishing pressure in a diverse range of fisheries. Many of these species are vulnerable to population collapse. Even though they are not directly targeted in commercial fisheries, they are caught as bycatch by longline, gillnet and trawl net fisheries within Chile’s Exclusive Economic Zone. Limited biological information is available to assess the potential impacts that anthropogenic disturbance may produce in populations of cartilaginous fishes, and this thesis aims to improve our knowledge of the taxonomy, life history and fisheries of cartilaginous fishes commonly caught in Chilean waters. The cartilaginous ichthyofauna in Chile was assessed through extensive surveys conducted in coastal artisanal, small to medium scale fisheries and larger semi-industrial oceanic fisheries. Here we review the taxonomy and fishery biology from a relatively basic level (checklist of living species, morphological descriptions and biology) towards complex conservation issues (distribution, habitat use and fishery mortality). However, these results represent the first attempt to quantify the abundance and distribution ranges of sharks, skates and chimaeras commonly caught along the Chilean coast. New biological and ecological data have complemented actual knowledge of cartilaginous fish dynamics in relation to fishing activity in the south-east Pacific Ocean. As part of this thesis, three new species have been added to the Chilean cartilaginous fauna (Chapter 2): the spinetail mobula Mobula japanica, the eastern Pacific black ghostshark Hydrolagus melanophasma and the thickbody skate Amblyraja frerichsi. The inclusion of these species improves identification methods of landing and fishery records. Each of these species addresses a different issue within the national fisheries context and in spite of none of these species is considered a ‘commercial resource’, all were obtained as bycatch arising from the targeting of other fishes. The occurrence of sharks, skates, rays and chimaeras in Chile is revised and information on the functional biodiversity provided (Chapter 3). A review of the primary literature on cartilaginous fishes, together with new information suggests that 106 species occur in Chilean waters; comprising 56 sharks, 30 skates, 13 rays and 5 chimaeras. Ninety-three species were confirmed to be present through validated sighting and/or catch records from national fisheries and from fisheries-related research cruises of which 30 species were encountered rarely. Also, the cartilaginous fish community in the continental shelf and slope off Chile was described, based on abundance and species richness from 20 species (9 sharks, 10 skates and 1 chimaera) caught in a 3,500 km2 trawl fisheries operation area. Additionally, the reproductive biology of the three main elasmobranchs that are target species were reported (Chapter 4): yellownose skate Zearaja chilensis, shortfin mako Isurus oxyrinchus and blue shark Prionace glauca. In the case of Z. chilensis, 5,477 specimens were analysed from three localities in South Chile and in a similar effort, 1,241 Isurus oxyrinchus and 1,153 Prionace glauca were analysed from a single locality in North Chile. Despite the fact that these species are caught in two fisheries that use different fishing gear and fish different zones, all three species experience one concerning similarity: over 80% of the total catch comprise immature specimens. This trend may compromise the stock integrity if intrinsic vulnerability and probable long-life spans of cartilaginous fishes are considered. Consequences for the species’ survival and possible signs of a fishery collapse must be reviewed in the short-term by management authorities to ensure the sustainability these fisheries. In the case of the shortfin mako, a pupping, and possibly a nursery area in the fishing zone was found which increase the urgency of management, such as the introduction of a minimum catch sizes, conservative fishing quotas and delimitation of fishing areas.Overall, the current thesis has expanded our knowledge of regionally important populations of cartilaginous fishes, and provides a biological baseline to define priorities for management and conservation.