California Sea Grant College Program

I tested the hypothesis that temperature, salinity, and dissolved oxygen influence the distribution and abundance of elasmobranchs (sharks, skates, and rays) in Tomales Bay, California. I captured elasmobranchs monthly (September 1990-April 1992) on longlines set in the bay. A Poisson regression under generalized least squares was used to determine that temperature and salinity were the most important factors determining the distribution and abundance of the three most commonly captured species; bat ray, Myliobatis californica, leopard shark, Triakis semifasciata, and brown smoothhound shark, Mustelis henlei. Females of all three species were more abundant than males throughout the bay, and were most abundant in the warmer more saline inner bay. All three species left Tomales Bay in late fall as water temperatures in the bay dropped below 10-12°C, and returned in early spring after temperatures rose above 10°C.

The oxygen consumption of bat rays, Myliobatis californica, was determined at 8, 14, 20, and 26°C. Mean, standard, mass-independent metabolic rates (MO2 in mg O2 •kg-0.67 •h-1) of unacclimated rays at each temperature were significantly different from one another. MO2 was extremely temperature sensitive from 20-26°C (Q10 = 6.62), and moderately sensitive at the upper (20-26°C: Q10 = 1.85) and lower (8-14°C: Q10 = 2.23) temperature ranges.

The effects of four temperatures (8, 14, 20, 26°C) on blood oxygen equilibrium curves of the bat ray, Myliobatis californica, was investigated. Blood oxygen equilibrium curves at all temperatures were hyperbolic (n50 < 2), with high affinity (low P50), large Bohr factors (0 = -0.45 to -0.52), high blood oxygen carrying capacity (CBO2 = 6.8 to 9.06), and very large non­-bicarbonate buffering capacity (β = -14.25 to -16.43). CBO2, 0, and β are among the largest measured in a poikilothermic elasmobranch. Affinity decreased with increasing temperature except at 26°C. The affinity shift at 26°C and a concomitant 25% drop in CBO2 at this temperature suggests a hemoglobin heterogeneity. Blood oxygen equilibrium curves were temperature insensitive betw en 8-14°C and 20-26°C and most sensitive between 14-20°C, the temperature range in which bat rays are most commonly found in Tomales Bay. Bat rays appear capable of tolerating anoxia and hypercapnia (low P50, n50, and high β), and sustaining high activity levels (high CBO2, 0, and β) by virtue of their unique hematological characteristics.