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Groundfish Assessment

Reproductive Biology, Spawning Season, and Growth of Female Rex Sole in the Gulf of Alaska

Rex sole (Glyptocephalus zachirus) have a wide distribution throughout the North Pacific, ranging from central Baja California to the western Bering Sea. Although rex sole are an important species in the commercial trawl fisheries off the U.S. West Coast, knowledge of their reproductive biology is limited to one study off the Oregon coast that analyzed ovaries with gross anatomical methods. The largest commercial harvest of rex sole occurs in the Gulf of Alaska (GOA) where they are managed as a distinct flatfish unit with species-specific assessments of biomass and acceptable biological catch estimates; however, there is no information on length or age at maturity to incorporate into analytical stock assessments. This study was initiated to determine reproductive and growth parameters specific to rex sole in the GOA stock. Female rex sole (n = 594) ranging in length from 166 to 552 mm were collected opportunistically around Kodiak Island, Alaska, from February 2000 to October 2001. All ovaries were analyzed to determine the maturity stage using standard histological criteria. Year-round sampling of rex sole ovaries confirmed that rex sole are batch spawners with a protracted spawning season in the GOA. The spawning season for rex sole spanned at least 8 months, from October to May, which is a longer duration and in different months than previously estimated.

Female rex sole in the GOA had an estimated length at 50% maturity (ML50) of 352 mm, which is 46% greater than the estimated ML50 for the stock off the Oregon coast. At the same length that 100% of rex sole from Oregon waters were mature, only 15.8% of rex sole in the GOA are mature. The maximum age of collected female rex sole was 29 years. The estimated age at 50% maturity (MA50) for female rex sole in the GOA was 5.1 years and was similar to the MA50 for rex sole off the Oregon coast. The von Bertalanffy growth model in the GOA was significantly different than the model from the Oregon coast. Higher growth rates in the GOA correspond with differences in length at maturity and similarity in age at maturity between the two regions. Reproductive parameters determined in this study will aid the development of a species-specific assessment of fishing mortality and exploitable biomass, yielding a more accurate management model for the GOA stock of rex sole.

By Alisa Abookire

Age and Length at First Maturity of Northern Rockfish Caught in the Central Gulf of Alaska

  Female northern rockfish ovaries
Figure 6.  Examples of development stages in female northern rockfish ovaries; (top) mature ovary with migratory nucleus present in an oocyte measuring 550 mm in diameter and (bottom) immature ovary with oocytes developed to the vitellogenic stage (secondary yolk), 230 micrometers in diameter.

The northern rockfish, Sebastes polyspinus, ranges throughout the northeast Pacific Ocean and, as the second most abundant rockfish species in the GOA, is one of the top commercial rockfish species caught in Alaskan waters. In 2002, commercial trawl fisheries for rockfish were valued at $2.2 million in the GOA, with the northern rockfish fishery second only to Pacific ocean perch. Despite this species abundance and commercial value, detailed reproductive studies have not been conducted. One objective of this study was to describe the reproductive maturity of female northern rockfish and estimate the age and length at first maturity. Prior to this research, estimates of the length and age at 50% maturity for northern rockfish were based on visual observations of gonad maturity taken from a limited sample collected in 1996. Results of that study are used in the current GOA stock assessment for northern rockfish (Heifetz, et al. 2003). In order to improve the maturity parameters of the stock assessment model, this study determined the maturity stage of female northern rockfish at the histological level with samples collected over a 2-year period from areas near Kodiak Island and in the central gulf.

A total of 158 ovaries with corresponding otoliths and fish lengths were collected from female northern rockfish to estimate age and length at first maturity. The laboratory analyses were done at the Kodiak Fisheries Research Center. The author used standard histological and staining techniques. The diameter of the fifth largest oocyte on a randomly selected transect along the histological cross-section of the gonad was used as the criterion for evaluating the most advanced nonatretic oocyte and to determine the maturity stage of the ovary. Ovaries with oocytes in the migratory nucleus stage and the presence of postovulatory follicles were used as evidence of a mature ovary (Fig. 6 above). Lengths of fish from the sample ranged from 14 cm to 44 cm and ages ranged from 3 to 34 years. The age and length at first maturity were estimated at 8 years and 310 mm. This research shows the presence of intermediate or maturing oocytes in all months sampled, as well as oocytes at the vitellogenesis stage with eyed larvae present in the month of April and post parturition in May and June (Fig. 7 below). Samples collected in April had the highest number of mature ovaries with 40% of the samples (n = 43) having oocytes at stage four or higher, while samples taken in July and November samples only had oocytes developed to the vitellogenesis stage, indicating a period of rest after parturition.

Figure 7, see caption
Figure 7.  Frequency of the most advanced oocyte stage present in female northern rockfish for each month sampled. Sample sizes are shown above each bar.

The maturity parameters used in the stock assessment model for GOA northern rockfish reflect an estimated length at 50% maturity of 36.1 cm and age at 50% maturity of 13 years. These estimates are based on a sample size of 77 females collected off Kodiak Island in April of 1996. Northern rockfish caught off the coast of northern British Columbia, Canada, are reported to mature at 19-22 cm and 5-7 years of age, values similar to the maturity estimates reported in this study. The maturity estimates used in the stock assessment report are larger and older than the values reported for northern rockfish in this study and Canada, which could be due to the small sample size or the limited seasonal component of the sample as well as the fact that these evaluations were based on macroscopic classification in the field, rather than histological assessment of the maturity stage.

By Liz Chilton



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