Trophic interactions and distribution of some Squaliforme sharks, including new diet descriptions for Deania calcea and Squalus acanthias

Abstract

Squaliforme sharks are a common but relatively vulnerable bycatch in many deep water fisheries. Eleven species of squaliforme shark are commonly caught at depths of 200-1200 m on Chatham Rise, New Zealand, and their diversity suggests they might occupy different niches. The diets of 133 Deania calcea and 295 Squalus acanthias were determined from examination of stomach contents. The diet of D. calcea was characterised by mesopelagic fishes, and S. acanthias by benthic to pelagic fishes, but was more adaptive and included likely scavenging. Multivariate analyses found the most important predictors of diet variability in S. acanthias were year, bottom temperature, longitude, and fish weight. The diet of the nine other commonly caught squaliforme sharks was reviewed, and the spatial and depth distribution of all species on Chatham Rise described from research bottom trawl survey catches. The eleven species had a variety of different diets, and depth and location preferences, consistent with niche separation to reduce interspecific competition. Four trophic groups were identified, characterised by: mesopelagic fishes and invertebrates (Centroselachus crepidater, D. calcea, and Etmopterus lucifer); mesopelagic and benthopelagic fishes and invertebrates (Centroscymnus owstoni, Etmopterus baxteri); demersal and benthic fishes (Centrophorus squamosus, Dalatias licha, Proscymnodon plunketi); and a generalist diet of fishes and invertebrates (S. acanthias). The trophic levels of the species in each of the four groups were estimated as 4.18-4.24, 4.20-4.23, 4.24-4.48, and 3.84 respectively. The diet of Oxynotus bruniensis and Squalus griffini are unknown. The different niches occupied by different species are likely to influence their vulnerability to bottom trawl fisheries. Some species may benefit from fisheries through an increased availability of scavenged prey.

Figure 1. Locations on Chatham Rise of the research trawl tows (crosses), and tows where a) Deania calcea, and b) Squalus acanthias were caught (open circles) and non-empty stomach samples were obtained (filled circles).

Circle sizes are proportional to catch (max. 189 kg for D. calcea and 264 kg for S. acanthias) and stomach sample size (max. 51 for D. calcea and 49 for S. acanthias). Grey lines show the 200 m, 600 m, and 1000 m isobaths; SI, South Island of New Zealand; CI, Chatham Islands; MG, Mernoo Gap; MB, Mernoo Bank; VB, Veryan Bank.

Figure 2. Cumulative diversity of prey categories (solid lines) and 95% credible intervals (broken lines) in the analyses of diet of a) Deania calcea, and b) Squalus acanthias.

Dotted lines are the fitted curves from which asymptotic diversities were estimated.

Figure 3. Catch rate (median kg km⁻²) of squaliforme sharks on Chatham Rise in 0.3° latitude and longitude cells.

Cells shaded in the lightest grey were sampled but no catches of that species were made; cells shaded in successively darker grey had higher mean catch rates. Maximum mean catch rates: Centroselachus crepidater 686 kg km⁻²; Deania calcea 473 kg km⁻²; Etmopterus lucifer 12 kg km⁻²; Etmopterus baxteri 560 kg km⁻²; Centroscymnus owstoni 98 kg km⁻²; Centrophorus squamosus 100 kg km⁻²; Dalatias licha 52 kg km⁻²; Proscymnodon plunketi 13 kg km⁻²; Squalus acanthias 4630 kg km⁻²; Squalus griffini 10 kg km⁻²; Oxynotus bruniensis 9 kg km⁻². Grey lines show the 200 m, 600 m, and 1000 m isobaths.

Figure 4. Catch rate (kg km⁻²) of squaliforme sharks on Chatham Rise by species and depth.

The solid line shows the LOESS regression fitted to catch rate; broken lines indicate the 95% confidence intervals. The vertical lines above the x-axis indicate the location of catches of that species.