The longnose skate, Raja rhina, is common in the eastern North Pacific Ocean ranging from the Bering Sea to Baja California and occurs from inshore to a maximum of 1000 m depth. In the Gulf of Alaska (GOA), it has a maximum total length (TL) of 145 cm. A directed fishery for Raja spp off Kodiak Island, Alaska was initiated in 2003 and ended in 2005. An experimental fishery in Prince William Sound, Alaska was reinstated in 2009. The vulnerability of elasmobranchs to over exploitation from commercial fishing, either from bycatch or a directed fishery, is well documented.
Figure 2. Longnose skate vertebral thin section (reflected light) standard (unstained) method used for age estimates, aged at 22 years. Band pairs evident in the corpus calcareum and intermedialia.
Figure 3. Average total length-at-age (von Bertalanffy growth curves) and frequency histograms from (a) females and (b) males; GOA (96 females, 115 males), BC (73 females, 101 males), WC (225 females, 176 males)
Figure 4. Age at proportion mature (logistic curves) for females; GOA (n = 96, BC (n = 73), WC (n = 225).
The Age and Growth (A&G) program recently completed a study in collaboration with Vladlena Gertseva (Northwest Fisheries Science Center), Jacquelynne King (Pacific Biological Station), and David Ebert (Pacific Shark Research Center). This inter-agency and institutional collaborative study quantitatively compared growth and age/size at sexual maturity of the longnose skate across two large marine ecosystems, the GOA and California Current Ecosystem (CCE).
Thoracic vertebrae (n = 838) were collected along with sex, maturity and total length off the GOA, British CColumbia (BC) and the U.S. west coast states (WC) between 2001 and 2009, May to August, via research surveys and port sampling (Fig.1). Vertebrae were thin sectioned (≈0.3 to 1.5 mm thick) on the longitudinal plane and microscope slide mounted.
Growth bands were examined under a dissecting microscope with reflected or transmitted light with the addition of mineral oil for band enhancement. Opaque and translucent band pairs were interpreted as an annulus, 1 year of growth, from the corpus calcareum and intermedialia (Fig. 2). Ages were estimated from vertebrae prepared with the standard (unstained) thin sectioning method in this preliminary study (Fig. 2)
Reader-tester precision from all regions was: agreement [(±0) (4.0% to 35.2%)], CV (5.7 to 13.8) and APE (0.04 to 8.3) (+/-0 (plus or minus 0 years); CV (coefficient of variation); APE (average percent error). The length range for aged specimens and maximum age (26 years) are similar across all regions, while the Linf values for males, females and sexes combined vary considerably (Table 1).
The GOA male and female length distributions are skewed towards the greater lengths (> 1,000 mm TL) compared to BC and the WC. Male and female length distributions for BC and the WC are similar even with the WC sample size (n = 401) being more than double that of GOA and BC (Fig. 3).
Overall, male and female von Bertalanffy growth curves show a greater length-at-age for GOA compared to BC and the WC. The GOA and the WC male growth curves converge at an age of 19 years and length of 1,200 mm (Fig. 3).
Females from the GOA and BC mature at a younger age than the WC. For the GOA and BC, females reach 50.0% sexual maturity at 11 years and at 17 years for the WC (Fig. 4). Differences in maturity characteristics, maturing or fully mature, and maturity stage assignment may explain the disparity between the GOA and BC/WC.
Future research will include re-evaluating and synchronizing ageing criteria upon completion of an initiated 14C bomb derived age validation study with the inter-agency age comparison of the standard thin section method and expansion of a histological hematoxylin staining technique.
Environmental (e.g., bottom water temperature), oceanographic effects and potential influences on life history traits across the GOA and CCE ecosystems, including the ‘current break’ between the Alaska and California Current, will be evaluated.
Table 1. A comparison of longnose skate sampling data, age and von Bertalanffy growth parameters by region. Numbers in brackets are 95% confidence intervals for parameter estimates; GOA (Gulf of Alaska); BC (British Columbia); WC (West Coast); TL (total length); Max Age (maximum age); Linf (L infinity-theoretical asymptotic length); K (Brody growth coefficient); to (theoretical age at zero length); r2 (coefficient of determination).
