Life history, foraging ecology, metabolic rates and bioenergetics of the brown stingray, dasyatis lata

Abstract

Life history characteristics, diet, nursery habitat use and metabolic rates of brown stingrays, Dasyatis lata, were evaluated to determine their ecological impacts in Kāne'ohe Bay, Oahu, through a bioenergetics model. The maximum age based on analysis of vertebral samples (28 years) was among the oldest for any dasyatid aged to date. Edge and marginal increment analyses, as well as recapture of two stingrays previously marked with oxytetracycline, verified deposition of a single growth band per year. Growth characteristics varied between sexes, with females attaining larger sizes and exhibiting lower growth coefficients. Brown stingray life history characteristics are similar to other elasmobranchs in that they are long lived, grow slowly and mature at a late age. Stomach content, bulk and amino acid stable isotope analyses were used to assess the diet and habitat use of juvenile brown stingrays in Kāne'ohe Bay. A shift in stingray diet and an increase in relative trophic position were apparent with increasing size. Results indicate stingrays foraged within their Kāne'ohe Bay nursery for the majority of their juvenile lives before shifting to offshore habitats with the onset of sexual maturity. Underestimates of trophic position from amino acid analyses suggest that urea retention in elasmobranchs may have important implications for estimates of absolute trophic position determined using this method. Potential prey resources were portioned between stingrays and sympatric juvenile scalloped hammerhead sharks, Sphyrna lewini. Standard metabolic rates were estimated for juvenile brown stingrays through respirometry. Temperature and mass had significant effects on metabolic rates. The energy budget of juvenile stingrays was heavily weighted towards metabolism, which accounted for 68% of total consumed energy. A large metabolic demand coupled with very slow growth indicated potential limitation of food resources. Daily rations estimated from the bioenergetics model declined from a high of 2.72 %BW/d to 1.23 %BW/d with age. Results suggest the potential for strong top-down effects on prey populations due to stingray predation. Use of Kāne'ohe Bay as a nursery habitat for brown stingrays appears to be a trade-off between increased juvenile survival and a late age at first maturity due to slow growth rates.