Life History, Abundance, and Distribution of the Spotted Ratfish, Hydrolagus colliei

Size at maturity, fecundity, reproductive periodicity, distribution, and abundance were estimated for the spotted ratfish, Hydrolagus colliei, off the coast of California, Oregon, and Washington (USA). Skeletal muscle concentrations of the steroid hormones testosterone (T) and estradiol (E2) predicted similar, but slightly smaller sizes at maturity than morphological criteria. Stage of maturity for males was estimated identically using internal organs or external secondary sexual characters, thus allowing non-lethal maturity assessments. Peak parturition occurred from May through October, with increased concentrations of E2 and progesterone (P4) in skeletal muscle of females correlating with ovarian recrudescence during November through February. Extrapolation of the hypothesized 6 to 8 mo egg-laying season to observed mean parturition rates of captive specimens yielded an estimated annual fecundity of 19.5 to 28.9 egg cases. Differences in fecundity among higher taxonomic classifications of chondrichthyans were detected, with chimaeriform fishes more fecund than myliobatiform, squaliform, and rhinobatiform fishes. Delta-lognormal generalized linear models (GLMs) and cluster analysis indicated the presence of two distinct stocks of H. colliei on the U.S. West Coast. Abundance of the continental slope, and northern continental shelf and upper slope populations did not vary between 1977 and 1995, but increased from 1995 to 2006. Abundance trends in the southern shelf and upper slope region were not as straightforward, with increasing abundance from 1977 to 1986, and lesser abundance thereafter, with the exception of an increase between 1992 and 1995. Although the life history, movement patterns, and aggregative behavior of H. colliei indicated that it may be vulnerable to population depletion by excess fisheries mortality, temporal abundance trends indicated that their population size has increased significantly within the last decade. The paradigm that all chondrichthyans are particularly susceptible to exploitation, therefore, may not apply to chimaeroids. The hypothesis that the dorsal-fin spine of H. colliei is a reliable structure for age estimation was tested by analyzing growth characteristics and imageing with polarized light microscopy and micro-computed tomography. Variation among individuals in the relationship between spine width and distance from the spine tip indicated the technique of transverse sectioning may impart imprecision and bias to age estimates. The number of growth band pairs observed by light microscopy in the inner dentine layer was not a good predictor of body size. Mineral density gradients, indicative of growth zones, were not observed in the H. colliei dorsal-fin spine, but were present in hard parts used for age determination of the Patagonian toothfish (Dissostichus eleginoides), roughtail skate (Bathyraja trachura), and spiny dogfish (Squalus acanthias). The absence of mineral density gradients in the dorsal-fin spine of H. colliei decreases the likelihood that the bands observed by light microscopy represent a record of growth with consistent periodicity