Estimation of key population parameters and MSY-based reference points for sidestripe shrimp (Pandalopsis dispar) in the Fraser River Delta, British Columbia
Introduction
Reference points are critically important for fisheries management, as they allow us to evaluate the stock status for possible management actions. Commonly used reference points are based on maximum sustainable yield (MSY), which fully account for all components of production, such as growth, survival and recruitment (Duplisea, 2013). However, estimation of these key parameters is difficult (Hoenig et al., 2016). It is especially a challenge to estimate the stock-recruitment relationship, as the nature of this relationship is seldom obvious from the available data (Sharma et al., 2019). The current study presents some novel ways to estimate these parameters and construct stock-recruitment relationships for the Sidestripe shrimp (Pandalopsis dispar Rathbun 1902) population of the Fraser River delta in the Strait of Georgia, British Columbia.
Crustaceans grow in size incrementally by shedding their hardened shell to uncover the soft and larger shell underneath through moulting. In common with most other shrimp species, Sidestripe shrimp growth is discontinuous and poorly understood (Jensen, 2014; Bergstrom, 2000). Male and female Pandalid shrimp in general have different growth patterns whereby males continue to grow and moult throughout the year while females stop moulting while carrying eggs (Butler, 1964; DFO, 1999).
Sidestripe shrimp occurs in the northeast Pacific Ocean from the Bering Sea to Oregon Coast, and are primarily found in depths between 90 and 201 m on soft mud substrates (Butler and Boutillier, 1983). Sidestripe shrimp is the second largest shrimp in British Columbia after the Spot Prawn (Pandalus platyceros). Maximum size of males is 31.0 mm carapace length (CL) and 182 mm total length (TL). Maximum size of females is 36.3 mm CL and 208 mm TL. They are easily distinguished from other shrimp species by their long antennules and striped abdomen (Butler, 1964, 1980; Butler and Boutillier, 1983). Sidestripe shrimp is short-lived protandric hermaphrodite like all other Pandalid shrimps (Bergstrom, 2000). They spawn in late fall, and eggs hatch in March or April the following year, after being brooded by females throughout the winter (Butler, 1964, 1980). Larvae metamorphose into immature males by July or August on the same ground inhabited by older age groups after several months of pelagic life. Males mature in their second year of life, and start the process of changing into females in the spring of the third year (Butler, 1980). Females reach sexual maturity by the autumn of their third year. After releasing hatched eggs in March or April, these females generally die.
Sidestripe shrimp is commercially exploited by bottom trawl gear primarily in the inshore waters of British Columbia. In the Fraser River delta (Fig. 1), they are harvested together with two smaller Pink shrimp species, Smooth Pink shrimp (Pandalus jordani) and Spiny Pink shrimp (Pandalus borealis) (DFO, 2019). The fishing season is between June 1 and March 31, but commercial shrimp trawl fishing has been closed in the Fraser River delta since 2017 due to low Pink shrimp abundance (DFO, 2019). Fishery-independent trawl surveys have been conducted annually since 1998 to estimate shrimp biomass for setting of annual quotas (Boutillier and Joyce, 1998; Boutillier et al., 1999; DFO, 2012, 2019). Biological data, such as length, weight and life stage of sampled individuals, have been collected during these fishery-independent bottom trawl surveys, providing valuable information. We adopted novel approaches to study the growth, mortality and recruitment of Sidestripe shrimp, using these biological data.
Construction of a growth model requires size-at-age information. Compared to finfish, age determination in crustaceans is challenging given the moulting of the exoskeleton (Chang et al., 2012). Conventionally, length-frequency analyses are used to obtain age information for crustacean species (Kilada and Driscoll, 2017), although, in recent years, growth bands from transverse sections of the eyestalk or gastric mill ossicles have been used for ageing (Leland et al., 2011; Kilada et al., 2012). Shrimp mortality rates are usually estimated using length-frequency analyses (Mace III and Rozas, 2015) or mark-recapture studies (Knudsen et al., 1996; Mace III and Rozas, 2015). Meaningful stock-recruitment relationships are often not detected for shrimp populations, although many studies have been conducted in this regard (Hannah, 1995; Cadrin et al., 2004).
We developed a new and simple way to estimate Sidestripe shrimp age, taking advantage of the unique features of their life history. We used identified life stages to determine hatching (birth) dates of sampled individuals, deriving their ages in days at the sampling time. We estimated the natural mortality rate by comparing the abundance of females as estimated in one survey year with the abundance of transition stage (Sex 2) and female stage (Sex 3) shrimp as predicted based on the estimated abundance of mature males (Sex 1) in the previous survey year and the amount of commercial removals. We also constructed stock-recruitment functions based on the correlations between spawned-out biomass and subsequent recruitment.
Constructions of growth and stock-recruitment functions together with estimates of natural mortality rates provide the biological basis for the estimation of MSY-based reference points. We estimated MSY, spawned-out biomass corresponding to MSY, and fishing mortality rate to achieve MSY, using an equilibrium approach through projection as shown by Duplisea (2013). These reference points may be used to fulfill the aim of stock assessment, which is to provide the scientific basis for management measures (Sharma et al., 2019).
Section snippets
Materials and methods
We first describe how the data were obtained and how the growth and length–weight (L–W) models were constructed. Then, we estimate the abundance of Sidestripe shrimp at the time of the surveys and the fishery removals at various sex stages. Subsequently, we estimate the natural mortality rate, and build stock-recruitment models. Finally, we derive MSY-based reference points, using the growth, L–W and stock-recruitment models, and the natural mortality rate.
Results
The growth and L–W models appear to fit the data reasonably well (Fig. 2, Fig. 3). The parameters of the two models are all highly significant (p < 0.001), and their estimates are shown in Table 2. Carapace length distributions appear to have two modes in most years, one around 20 mm and the other around 27 mm (Fig. 4). The carapace lengths for these two modes correspond well to the mean sizes of Sex 1 and Sex 2–3 Sidestripe shrimp at the times of the surveys. The abundance trend for Sex 1
Discussion
Fishery-independent survey data provide valuable measures of abundance, as sampling is scientifically designed and standardized (Gunderson, 1993). In addition, biological information on target species can be obtained during the survey. However, long-term fishery-independent data are often not present for many fisheries despite their high importance (Dennis et al., 2015). We have a reasonably long time series (21-years) of fishery-independent survey data on the Sidestripe shrimp population in the
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
We would like to thank Dominique Bureau and Andres Araujo for their helpful reviews of the manuscript, and are indebted to the two anonymous reviewers for their constructive comments, which helped improving the quality of this paper. We are also grateful to Hai Nguyen for discussions about the Sidestripe shrimp fishery, data, and modelling.
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