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Resource Ecology & Ecosystem Modeling Program

Maturity of Sebastes spp. in the Aleutian Islands: Filling Critical Life-History Data Gaps for Data-Poor Commercially Important Rockfishes

figure 2,see caption Figure 2.  Macroscopic (a) and histological (b) view of an ovary from a mature POP collected in October 2010.  Note the yolk globules (advanced vitellogenesis) from the histological view.

figure 3, see caption Figure 3.  Macroscopic (a) and histological (b) view of an ovary from a large-sized blackspotted rockfish collected in April 2010.  There was no evidence of vitellogenesis in the histological view.

Estimates of maturity, in particular the proportion of a population mature by age, are an important metric of fish populations and play a critical role in the formulation of fishing reference points and harvest specifications. Age-structured stock assessment models for rockfishes in the Bering Sea-Aleutian Islands (BSAI) management region rely upon maturity data from the Gulf of Alaska (GOA) due to the lack of data in the BSAI. Misspecification of fishing mortality rates would occur if differences existed between the productivity of a species occurring in different regions, and species-specific maturity information will be required for more refined assessment methodologies.

The primary focus of this study was to obtain updated maturity information from females for five rockfishes occurring in the Aleutian Islands region: Pacific ocean perch (POP; Sebastes alutus); northern rockfish (S. polyspinis); blackspotted rockfish (S. melanostictus); rougheye rockfish (S. aleutianus); and shortraker rockfish (S. borealis).

Field sampling for ovaries and otoliths occurred during spring, summer, and fall 2010 aboard commercial and AFSC-chartered vessels. After histological examination of ovaries and ageing of specimens, age and length at 50% maturity was estimated for Pacific ocean perch (8.7 yrs at 32.4 cm) and northern rockfish (6.6 yrs at 27.7 cm). The presence of yolk (vitellogenesis) was the determining factor when assigning fish as mature vs. immature.

Fall sampling appeared to be ideal for POP and northern rockfish for estimating maturity. Summer POP samples were also included in the maturity analysis. Visual macroscopic inspections of ovaries were also recorded during the fall sampling period for both species (Fig. 2). Macroscopic observations recorded in this study generally matched subsequent histological analysis for both larger specimens and smaller specimens when defining mature vs. immature specimens.

Due to a lack of smaller-sized specimens caught and sampling during periods of reproductive inactivity, maturity estimates for blackspotted, rougheye, and shortraker rockfish could not be obtained. Throughout the sampling periods, blackspotted rockfish within our desired size range exhibited macroscopic and histological characteristics as immature, although many of the larger fish may have been resting or reproductively inactive mature fish (Fig. 3). This was also observed for rougheye and shortraker rockfish.

For these species, the seasonal reproductive cycle may be more compressed into the late fall to spring period, and further sampling throughout the year should be conducted to evaluate this hypothesis. Assessment models and harvest recommendations, however, will be improved by obtaining region-specific maturity information from the Bering Sea-Aleutain Islands area for Pacific ocean perch and northern rockfish and periodically updating the maturity information to monitor any temporal trends.

By Todd TenBrink

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