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Description
Mortality, fecundity, and size at maturity are important life history traits, and their interactions determine the evolution of life history strategies (Roff, 1992; Stearns, 1992; Charnov, 2002). These same traits are also important for population dynamics models (Hunter et al., 1992; Clark, 1999). It is increasingly important to accurately determine Greenland halibut (Reinhardtius hippoglossoides) life history traits and to correctly assess the status of its stocks because low recruitment or low biomass estimates have led to catch restrictions in the Bering Sea and Aleutian Islands (Ianelli et al. (1)), the Northeastern Arctic (Adlandsvik et al., 2004), and the Northwest Atlantic (Bowering and Nedreaas, 2000).
Mortality has been estimated for stocks of Greenland halibut from the Northwest Atlantic (Bowering, 1983) and from the Bering Sea (Ianelli et al. (1)) from population age structures where a maximum age near 20 years has been assumed. However, recent age validation (Treble et. al (2)) and a new aging technique (Gregg et al., 2006) indicate that Greenland halibut may have a lower mortality rate and live longer. Fecundity estimates (Gundersen et al., 2000) and length at maturity estimates (Morgan et al., 2003) vary by geographic area and year for Greenland halibut in the Atlantic. Fecundity of Greenland halibut from the Bering Sea has been estimated once (D'yakov, 1982), and no estimates for Greenland halibut maturity at length have been reported from Alaskan waters.
A significant and positive relationship between natural mortality rate (M) and annual reproductive effort, as measured by gonadosomatic index (GSI=ovary weight/somatic body weight), was found for 28 fish stocks of a variety of species (M=1.79 x GSI, [r.sup.2]=0.75) (Gunderson, 1997). This GSI-M relationship can provide an estimate of M independent of age data, and is desirable for Greenland halibut because age data are still controversial for this species. The objectives of this study are to estimate annual fecundity, length at 50% maturity, and the rate of instantaneous natural mortality (M) by using GSI for Greenland halibut from the Bering Sea and Aleutian Islands.
Materials and methods
Ovaries were collected by fishery observers and scientists from the National Marine Fisheries Service (NMFS) in the Bering Sea and Aleutian Islands (Table 1, Fig. 1). Fork length (cm) and somatic weight (kg) (ovaries and stomach contents removed) were recorded at sea. Total weight was approximated by adding somatic weight and the estimated fresh ovary weight.
[FIGURE 1 OMITTED]
Ovaries were removed and placed in a 10% formalin solution buffered with sodium bicarbonate. Twenty-eight ovaries were weighed ([+ or -]2 g) before being placed in formalin. The fresh weight was later compared to formalin weight to determine a conversion factor when fresh weight was not available. Ovaries were removed from the 10% buffered formalin, blotted dry, and weighed ([+ or -]0.001 g). When possible, a whole cross section was removed from one lobe of an ovary for histological analysis. If the ovary cross section was too large to fit on a slide, a wedge-shaped sample was removed which included tissue from the center of the ovary to the ovarian wall. Ovary tissue samples were embedded in paraffin, sectioned at 4 [micro]m, and stained with hematoxylin and eosin. All 56 specimens smaller than 40 cm were obviously immature because of their very small ovaries; therefore histological examination was completed on only six of these specimens to verify that they were immature.
Each slide was analyzed for the oocyte stages present and for postovulatory follicles (POFs) according to the descriptions and images from Gundersen (2003) (Fig. 2). Maturity stages were assigned according to Gundersen et al. (2003).
[FIGURE 2 OMITTED]
Length-GSI relationship and instantaneous rate of natural mortality
For the GSI-M relationship, Gunderson (1997) defined GSI... |
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