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Groundfish & Shellfish Assessment Programs

Reproductive Biology of Pacific Ocean Perch, Sebastes alutus, in the Gulf of Alaska

figure 1, see caption
Figure 1.  A histological section of a Pacific ocean perch eyed embryo.

Pacific ocean perch are the most abundant rockfish in the Gulf of Alaska and the most commercially important rockfish in this region. The stock assessment of this species in the Gulf of Alaska utilizes an age-structured model as the primary assessment tool. This model incorporates maturity estimates which directly influence the estimation of stock biomass and a very small change in this parameter can have a significant impact on the determination of total allowable catch. Therefore, it is critical to have accurate and up to date data on the reproductive parameters of this rockfish species.

The values for age and length at maturity of this species currently utilized in the stock assessment are derived from a study predominantly employing visual techniques to assess maturity status and oocyte development. The objective of this study was to update and re-examine the reproductive biology of this species utilizing histological techniques to examine the seasonality and length and age at maturity of this species within the Gulf of Alaska.

Pacific ocean perch samples were collected opportunistically throughout the year during AFSC and ADF&G scientific surveys, from the North Pacific Groundfish Observer program, and from dedicated charters on the fishing vessel Goldrush. Samples were collected predominately from the central Gulf of Alaska, but some samples were taken from both the eastern and western Gulf, particularly in the late summer months.

Fish lengths, weights, and ovary weights were measured and ovarian and otolith samples were collected. Ovarian samples were sectioned and stained with hematoxylin and eosin utilizing standard histological techniques (Fig. 1). These sections were examined to determine the maturity state of each fish sampled. The maximum ova (or embryo) diameter (MOD) was measured for each sample and a gonadosomatic index (GSI) was calculated by dividing the ovarian weight by the total weight of the fish. The MOD and GSI were utilized to examine the seasonality of the reproductive cycle.

figure 2, see caption
Figure 2.  Mean monthly gonadosomatic index (GSI) and maximum ova diameter (MOD) values.  Error bars are standard error of the mean.

The results from the GSI show an increase in the size of ovary through the months of March, a decrease in May as parturition is occurring, and a sharp drop-off in June after parturition has occurred (Fig. 2). The mid-range value and high variability of this value during May reflect that fish were captured both prior to and after parturition. The mean monthly MOD also follows the same trend, but since it only includes oocytes that are developing, the value for MOD peaks in May.

Values for MOD in June and July were absent due to the resting stage of Pacific ocean perch oocytes during this period (Fig. 9). The reproduction of this species was found to be highly synchronous with a prolonged period of development between the beginning of vitellogenesis (yolk deposition) in July and August to parturition in May.

figure 3, see caption
Figure 3.  Logistic maturity curve for age of Pacific ocean perch.

Due to the difficulty of distinguishing maturing and resting-stage fish during the late summer months, fish collected during July and August were not included in the length at maturity calculation. The smallest mature fish was 308 mm fork length (FL) and the largest immature fish was 430 mm FL. Length at 50% maturity was calculated to be 333 mm FL. The youngest mature fish was age 7 and the oldest mature fish was age 58; the ovary of this fish was undergoing active atresia (degeneration). Only two immature fish were older than 17 years (23, 58). Age at 50% maturity was calculated to be 8.2 years (Fig. 3).

The management of Pacific ocean perch in the Gulf of Alaska is dependent upon accurate estimation of life history parameters including length and age at 50% maturity. This study found smaller and younger age and length at maturity estimates which may have important implications for stock assessment for this species in the Gulf of Alaska.

By Christina Conrath and Brian Knoth

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