Temporal variability in the reproductive parameters of deepwater rockfishes in the Gulf of Alaska
Introduction
Rougheye rockfish, Sebastes aleutianus, and shortraker rockfish, S. borealis, are deepwater rockfishes that occur on the continental shelf edge and slope in the Gulf of Alaska (Allen and Smith, 1988; Orr and Hawkins, 2008). These two species are characterized by large size, late maturity, and relatively high fecundity values characteristic of the equilibrium life strategy defined by Winemiller and Rose (1992) that is common in temperate marine fishes. Large body size enhances survivorship during periods of unfavorable conditions, allowing organisms to store energy for future reproductive events, whereas high fecundity allows for higher recruitment during years with conditions that favor survivorship and growth of larvae (Winemiller and Rose, 1992). The lack of life history data combined with the difficulty in ageing shortraker rockfish make it difficult to study patterns of recruitment for this species. The age structure of rougheye rockfish in this region suggest that they have episodic periods of higher recruitment typical of this life history strategy, and it is likely shortraker rockfish have similar recruitment patterns (Echave and Hulson, 2019; Shotwell and Hanselman, 2019). Rockfish of the genus Sebastes are live bearers with internal fertilization (Kendall and Lenarz, 1986). Although fecundity tends to be lower than other similar teleost species that occur in the same environment, fecundity of rockfish species within the Alaska region still ranges in the tens to hundreds of thousands (TenBrink and Spencer, 2013; Conrath, 2019).
In addition to internal fertilization, rockfish of this genus have other unique reproductive characteristics that contribute to this equilibrium life history strategy. The reproductive mode of members of the Sebastes genus is true viviparity, with embryos receiving additional maternal nutrition beyond what is provided from the yolk sac during gestation (Boehlert et al., 1986). Many rockfish species have immature individuals that undergo abortive maturation, where reproductive development is initiated but oocytes are later resorbed (Nichol and Pikitch, 1994; Hannah and Parker, 2007; Conrath and Knoth, 2013). This process is distinguished from skipped spawning, where mature adults skip a spawning season. Skipped spawning has also been documented for these species as well as other members of this genus in the Alaska region (Conrath, 2017, 2019). In addition, maternal effects appear to be important for this genus with documented increases in relative fecundity (Beyer et al., 2015), increased larval quality (Berkeley et al., 2004), decreased occurrence of skipped spawning (Conrath, 2017), and earlier parturition (Sogard et al., 2008; Rodgveller et al., 2011) observed with increasing size or age of the fish. All these characteristics increase the ability of these species to take advantage of years with favorable conditions and survive years with unfavorable conditions.
Both of these species are managed in the Alaska Region by the North Pacific Fishery Management Council. Age at maturity estimates are important components of stock assessments. Rougheye rockfish are managed as a complex with blackspotted rockfish, Sebastes melanostictus. In the Gulf of Alaska these two species are modeled together and have combined catch limits but this may be problematic due to differences in growth and reproductive parameters (Shotwell et al., 2014; Conrath, 2017). An age-structured population model is used as the primary assessment tool for this complex (Shotwell et al., 2014). Maturity at age estimates are used in this model and influence the estimation of mature female stock biomass. Due to difficulties in aging shortraker rockfish an age-structured model is not used in the assessment of this species. Shortraker rockfish are managed as a Tier 5 species and catch limits are derived from estimates of biomass and natural mortality.
Fecundity and skipped spawning information is not currently incorporated into stock assessment for these species, but changes in these parameters provide important insight into how reproductive success may be changing through time in response to environmental conditions. Fecundity has been correlated with prespawning energy reserves and some species downregulate fecundity in response to energy reserves during development (Rijnsdorp, 1991; Skjaeraasen et al., 2006; Kennedy et al., 2009). Skipped spawning is also frequently associated with food availability, fish condition, and energy reserves (Rideout and Rose, 2006; Skjaeraasen et al., 2012; McBride et al., 2015). Determining how fecundity data and rates of skipped spawning should inform stock assessment for rougheye rockfish and shortraker rockfish is complicated by both a lack of historical data and information on temporal or spatial changes in these parameters.
A better understanding of if and how age at maturity, fecundity, and skipped spawning rates vary with time will inform how these parameter estimates should be incorporated into stock assessment efforts. A comprehensive approach examining all of these parameters will provide information on both the success and failure of reproductive efforts. A recent study of the reproductive biology of rougheye and shortraker rockfishes in the Gulf of Alaska collected from 2008 to 2014 found that these species have an age at 50 % maturity of 19.6 years and a length at 50 % maturity of 45.0 cm for rougheye rockfish and length at 50 % maturity of 49.9 cm for shortraker rockfish. Age data was not available for shortraker rockfish. Skipped spawning rates were 37.4 % for rougheye rockfish and 60.0 % for shortraker rockfish and were related to the total length of the fish with larger fish being less likely to skip a spawning event (Conrath, 2017). It is unknown how size at maturity, fecundity, and rates of skipped spawning vary in time and therefore it is difficult to determine if these high rates of reproductive failure are a cause for concern. The objective of this study was to examine the reproductive parameters of these species in a different time period to better understand how consistent these parameters remain through time and how they may change in response to changing environmental conditions. During this study, rougheye and shortraker rockfish specimens were collected from the same geographic area as this earlier study in order to examine maturity, fecundity, and skipped spawning rates. In addition, the fecundity of specimens collected for the prior study were examined. These data will be used to comprehensively examine the reproductive success and failure of these two species during these two time periods.
Section snippets
Materials and methods
Shortraker and rougheye rockfish samples were collected offshore of Kodiak Island using a commercial bottom trawl net, during a scientific charter on the FV Gold Rush in December 2015. Each fish was weighed (g), measured for length (mm), and the ovary was removed and preserved in 10 % neutral buffered formalin. The sagittal otoliths were extracted and placed in a glycerol-thymol solution. A formalin-fixed ovary weight was taken once the samples were returned to the laboratory, and a section of
Size and age at maturity
During the December 2015 research cruise all rougheye and shortraker rockfish captured were sampled. Length at 50 % maturity of rougheye rockfish was calculated to be 447 mm (C.I. = 440–452 mm) and the age at 50 % maturity of rougheye rockfish was calculated to be 17.7 years old (C.I. = 16.4–19.4 years old). The age data was more variable than the length data and there were some old immature individuals but they tended to be smaller fish (Fig. 1, Table 3). Deep water rockfish otoliths are
Discussion
The reproductive output of a large number of teleost species varies temporally due to changes in a number of reproductive processes. Temporal changes in the age or size at maturity have been documented for a variety of species in a diverse array of locations (Harris and McGovern, 1997; Mangeni-Sande et al., 2019; Hunter et al., 2019). Interannual differences in fecundity (McElroy et al., 2016) and skipped spawning (Rideout et al., 2006; Skjaeraasen et al., 2015) have also been documented in
CRediT authorship contribution statement
Christina L. Conrath: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft. Peter-John F. Hulson: Methodology, Formal analysis.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors wish to thank K. Echave and K. Coutre for valuable assistance in the field and C. Rodgveller for assistance with the logistics and preparation of the field work associated with this project. In addition, we wish to acknowledge the crew and the captain of the FV Gold Rush for assisting in collecting samples during charter operations. We would also like to thank the AFSC Age and Growth laboratory for providing age and growth data for rougheye rockfish. This work was supported by the
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