Comparative feeding ecology of four sympatric skate species off central California, USA

Abstract

The big (Raja binoculata), California (R. inornata), longnose (R. rhina), and sandpaper (Bathyraja kincaidii), skates are commonly found on soft-bottom regions of the central California continental shelf and upper slope. The feeding ecology of this assemblage was compared to evaluate the degree of trophic separation among species, based on the results of previous speciesspecific diet studies. Specimens were collected from fishery independent trawl surveys conducted during September 2002-March 2003 at depths of 9-536 m. Using single and compound measures, diet composition of small (£60 cm TL) and large (>60 cm TL) individuals were compared within continental shelf (£200 m) and slope (>200 m) regions using traditional, multivariate, and novel techniques. Diet compositions within size classes were similar in both regions. Diet compositions between size classes generally differed, however, with fishes more important and crustaceans (especially shrimps and euphausiids) less important in the diets of large individuals. Crabs contributed substantially to skate diet compositions on the shelf, but were uncommon prey items at deeper depths, probably because of their relative scarcity in slope waters. Conversely, cephalopods were common prey items at slope depths, but were rarely ingested at shelf depths. The studied skate assemblage appears to consist primarily of generalist crustacean and fish predators that exhibit high dietary overlap at similar sizes. It is possible that resource competition among skates and groundfish species has been reduced because of considerable recent declines in the biomass of upper trophic level groundfishes. Skates may therefore play important roles in contemporary benthic food web dynamics off central California.

Feeding ecology of demersal elasmobranchs from the shelf and slope off the Balearic Sea (western Mediterranean)

“…On the CS, diet overlap was biologically significant among all the batoid species, except M. aquila, and between the skate R. clavata and the shark S. canicula. The lack of significant dietary differences among comparably-sized shelf skates suggests that inter-specific resource competition was not intense (Bizzarro et al, 2007). High values of overlap do not necessarily imply competition, except when resources are in short supply (Macpherson, 1977;Cartes, 1998).…”

Atlantic Bluefin Tuna (Thunnus thynnus) Diet in the Gulf of St. Lawrence and on the Eastern Scotian Shelf

“…Prey species were grouped by family and cumulative prey curves (CPCs) were created a posteriori to determine whether the sample sizes were sufficiently large to be used to describe prey abundance in both port locations. The CPCs were built by randomly resampling the stomachs 1000 times (Bizzarro et al, 2007) and plotting the mean cumulative number of identified taxa against the number of stomachs sampled (Ferry and Cailliet, 1996 …”

The artisanal elasmobranch fishery of the Pacific coast of Baja California Sur, Mexico, management implications

“…), and used to calculate a mean and standard deviation estimate for each sample (Colwell 2009). Linear regression was used to determine quantitatively whether the curve reached an asymptote, signifying an adequate number of samples (Bizzarro et al 2007b).…”