Biology and distribution patterns of some deepwater demersal fishes in the North Atlantic, with special reference to Antimora rostrata A study related to the project “Patterns and Processes of the ecosystems of the northern Mid-Atlantic”, MAR-ECO, 2001-2008

Abstract

The study seeks to increase our knowledge on deepwater demersal fish assemblages and the biological adaptations of Antimora rostrata. This widespread deep-living demersal teleost is essentially used as an example species. Managing deepwater resources and communities is especially difficult since our knowledge of the community structure, ecology and biology of individual species is limited, and fishery independent information is scarce. In this account focus is directed towards the use of longlines in describing deepwater assemblages. New information on the biology of A. rostrata is presented, and experimental and alternative ageing methods are explored. The study was an element of MAR-ECO, a field project of the Census of Marine Life (www.coml.org), studying the diversity and biology of biota along the mid-Atlantic Ridge (MAR). As part of the 2004 MAR-ECO expedition to the mid-Atlantic Ridge, the chartered Norwegian fishing vessel M/S Loran deployed longline sets across the ridge axis, sampling a depth range of about 4000 m. Overall, chondrichthyans dominated the catches on the MAR and contributed nearly 60% in terms of weight and numbers. Multivariate analysis (MDS) using species-by-station data indicated an assemblage distribution that related primarily to factors varying by depth and latitude. Grouping patterns of stations were not very pronounced, suggesting a gradual rather than abrupt change in species composition by depth or latitude. Catch rates peaked at the shallower stations in the Charlie-Gibbs Fracture Zone (CGFZ), and generally decreased with depth. Relatively large individuals dominated, and the overall mean weight was 2.4 kg. Average fish weight was higher in the sub-area sampled just north of the Azores archipelago than in the CGFZ sub-area. No depth-related pattern of individual fish size was found. The findings suggest that the use of longline is a valuable and possibly necessary tool for describing the larger scavenging fish assemblages on rough bottom at greater depths. Among the many different species caught frequently on longlines A. rostrata was analysed in more detail. Additional data from MAR, off Greenland and the British Isles were included, and used to exemplify adaptations of a deepwater species with respect to distribution and life history traits. Data on A. rostrata are presented from 42°N on the MAR to 65°N on the slope off West and East Greenland, along a northsouth range of approximately 1400 nm (~2600 km). The species occurred in the depth interval 670-3060 m in trawl and longline experiments sampling the depth range 400-4300 m. Length frequencies indicated both latitudinal and depth-related variation. Along the ridge abundance peaked between 1200 and 2700 m with generally low or no catches outside this range. Examination of growth patterns in transverse sections of otoliths suggested a strong cyclic zonation pattern in A. rostrata. Similarly, frequency distributions of fish length and otolith weight showed a number of modes, suggesting groups of specimens of similar size and age. Distances between modes were consistent and corresponded to what was expected based on the interpretation of the zonation pattern in otoliths. Assuming that the otolith growth zones counted represent annuli and that unbiased age readings was achieved, the growth of A. rostrata seemed linear with age for both otolith weight and fish length within the length range investigated. This corresponded to approximately 25 mg/y and 4 cm/y respectively. The oldest fish recorded was about 25 years old and sexual growth dimorphism, where females grow larger, older, and possibly faster, was suggested. The results indicate that more attention should be directed towards using frequency distributions, especially of otolith weight, for deriving information on age structure and population growth data for certain fish stocks. For A. rostrata, and many other deepwater species, greater efforts should be made to validate age determination methods.