ICES Marine Science Symposia

ICES mar. Sei. Symp., 199: 129-138. 1995 Offshore lobster (Homarus americanus) trap-caught size frequencies and population size structure D. S. Pezzack and D. R. Duggan Pezzack, D. S., and Duggan, D. R. 1995. Offshore lobster (Homarus americanus) trap-caught size frequencies and population size structure. - ICES mar. Sei. Symp., 199: 129-138. The hypothesis that size frequencies of trap-caught American lobsters (Homarus americanus) are due to size-related catchability and gear selectivity, rather than to population size structure, was examined using Canadian offshore lobster data. The offshore catches have a dome-shaped size distribution, a mean carapace length (CL) between 110 and 125 mm, and there are few lobsters smaller than 90 mm CL or larger than 170 mm CL. The relationship between the catch and the population size frequen cies was examined using a modeled population. The model showed that an unfished population would have a relatively flat size distribution with a wide range of sizes, and not the dome-shaped distribution. Movement data indicated that immature lobsters (<95 mm CL) are less mobile then mature sizes, and have a lower probability of encountering the traps. Very large animals (>170 mm CL) are underrepresented in the catch, owing to behavior and physical restrictions of trap size and design. Knowl edge of gear selectivity is basic to the interpretation of trap-caught size data. Size frequencies must be used with caution in lightly fished or newly exploited populations that have a wide range of sizes and the potential of catchability varying with size. D. S. Pezzack and D. R. Duggan: Department o f Fisheries and Oceans, Scotia-Fundy Region, PO Box 550, Halifax, NS, Canada B3J 2S7 [tel: (+1)902 426-2099, fax: (+1) 902 426-1862], Introduction hypothesizing relationships between stocks (Stasko and Pye, 1980). Size-frequency data are often the only data Knowledge of the age or size structure is important for available with which to study the early changes in a understanding and predicting how a population will newly exploited population. respond to fishing pressure. Direct measurements of The wide use of size-frequency data emphasizes the populations are rarely available and data from the com importance of knowing whether trap-caught animals are mercial fishery or research surveys must be used. In representative of the population. This article examines crustacean fisheries, size data are generally obtained the possible relationship between size frequencies in the from trap-caught samples which have inherent biases commercial trap catch and that of the underlying popu related to the physical design of the trap, fishing strategy lation using data from the recently exploited Canadian used, and the behavior of the animals, including offshore lobster fishery. The Canadian offshore fishery seasonal, size, maturity, and sexual differences in of the Gulf of Maine and Scotian Shelf (Fig. 1) started in catchability (reviews by Krouse (1989) and Miller 1972 and offers the opportunity to examine the dynamics (1990)). of the early phase of exploitation. Lobsters (Homarus americanus) cannot be aged di rectly and size (carapace length, CL) is used as a basis for the demographics of the population. Size-frequency Offshore lobster fishery and size data have been used to estimate exploitation rates frequencies in the commercial catch (Caddy, 1977; Campbell 1990), changes in levels of recruitment (Campbell, 1990), population reproductive The American lobster is distributed from Labrador, potential (Campbell and Pezzack, 1986; Harding and Canada, to North Carolina, USA. Though lobsters are Trites, 1988; Pezzack, 1989), and also as a basis for most abundant in the shallow nearshore environment, 130 D. S. Pezzack and D. R. Duggan ices mar. sd. Symp., 199 (1995) 47° NEW BRUNSWICK M onctory 45 43 Highly productive lobster grounds Lobster fishing grounds Known lobster distribution rorsair Canyon Figure 1. Known lobster distribution and fishing areas of the Scotian Shelf and eastern Gulf of Maine. Distributions are based on information from fishermen’s logbooks, tag returns, and groundfish trawl survey lobster by-catch. they are also found in exploitable concentrations in deep was expected to shift towards the smaller sizes as the (100-550 m) offshore areas from the southern Scotian slower-growing large individuals were removed, and a Shelf and the Gulf of Maine to slope regions off North smaller proportion survived to grow to the larger sizes. Carolina. The outer shelf and deepwater basin areas From the initial year of the fishery, the size-frequency provide oceanographic conditions with year-round tem distribution of the catch was dome-shaped with few peratures suitable for growth and reproduction (Pezzack immature animals (less than 95 mm CL). The size struc and Duggan, 1985,1987). American trawlers began fish ture remained constant between 1973 and 1986 with no ing offshore lobsters on Georges Bank in the 1950s significant changes in the overall frequency (Pezzack (Skud, 1969; Fogarty and Idoine, 1986). Canada estab and Duggan, 1988) even though the fishery removed an lished a trap-based offshore fishery in 1972, which fishes average of 5601 per year (Fig. 2). In contrast, Skud the deep basins of the eastern Gulf of Maine, the slope (1969) observed a dramatic shift in the size frequency of south of Browns Bank, and the northeast portion of trawl-caught lobsters off southern New England within a Georges Bank. 9-year period (1956-1965) and a progressive shift to At-sea samples were taken during the first two years smaller sizes over 3 years of sampling (1965-1967). This of the fishery (1972-1973) and a regular sampling pro fishery used the less selective bottom trawl, and levels of gram was established in 1977 to monitor size frequency fishing and incidental mortality are unknown. and mean size (Pezzack and Duggan, 1985,1987) in the The size frequency of offshore lobster differs mark belief that changes over time could be detected which edly from those of the long exploited coastal fisheries. would reflect exploitation rate and changes in the popu The mean carapace length of commercially caught off lation. With exploitation the population size structure shore lobsters ranges from 110 mm CL in deepwater ic e s mar. Sei. Symp., 199 (1995) Offshore lobster trap-caught size frequencies and population size structure 131 0.04 mortality for non-molting and molting animals, respect ively. The simulation was run for a 50-year period. The 1973 1986 model assumed constant recruitment and mortality, and £ 0.03 no immigration or emigration. The model serves as a best estimate of the theoretical size structure for use in the generation and testing hypothesis. Similar modeling approaches have been used in Yield/Recruit calculations (Fogarty and Idoine, 1988; Shotton, 1989) and studies of 0.01 crustacean growth strategies (Hartnoll, 1983). With only natural mortality, the numbers in the modeled population declined slightly with increasing 75 85 95 105 115 125 135 145 155 165 175 size up to approximately 100 mm CL (Fig. 4). At sizes Carapace Length (mm) greater than 100 mm CL the proportion molting declines Figure 2. Comparison of the size distribution obtained from at- sea sampling of commercial trap-caught lobsters from Corsair (Fig. 4) and there is an accumulation of lobsters at the Canyon (Georges Bank) during 1973 (the first year of the larger sizes. The proJected size structure resembles fishery) and 1986. those proposed by Hartnoll (1983) for several crab species. basins of the Gulf of Maine to 125 mm CL in the offshore The shape of the distribution and the degree of canyons of Georges Bank (Pezzack and Duggan, 1985, accumulation of the larger sizes is dependent upon the 1987). The offshore catches have few pre-recruits or relationship between size and proportion molting. The newly recruited animals (Fig. 3). By contrast the mean intermolt periods greater than 10 years (proportion sizes in the heavily exploited coastal fisheries of south molting less than 0.10 per year) were calculated from the western Nova Scotia are generally less than 90 mm CL tagging data (Pezzack and Duggan, 1990). These long (Campbell, 1990); there is an abundance of pre-recruits intermolt periods lead to the large accumulation of (70-80 mm CL) and few lobsters larger than 120 mm lobsters larger than 140 mm CL. However, these long CL. intermolt periods are not supported by laboratory obser vations that indicate that the maximum intermolt period is not greater than 6 years (S. Waddy, Department of Offshore population size structure Fisheries and Oceans, St Andrews, NB, pers. comm.). All information on the offshore lobster population is Unrealistically long intermolt periods would result if derived from trap catches (Pezzack and Duggan, 1985, catchability varies with size. The anniversary method 1987). Independent estimates of the population size used (Hancock and Edwards, 1967) compares the frequency are impossible, because it is inaccessible to number of tag returns from molted and non-molted direct observation by diving. It is therefore necessary to individuals in a size group, and assumes that the prob use indirect methods, such as modeling, to examine the ability of recapture is the same for both groups. If, relationship between the catch and population size however, individuals that molted to a larger size have a structure. lower probability of recapture, the proportion molting A lobster population was simulated using growth data would be underestimated. Knowledge of size-related to obtain a theoretical population size structure with catchability would allow correction for this. which to compare the sample data. Molt increment and A flatter distribution, with a reduction in the accumu proportion molting at each size (Fig. 4) were obtained lation at larger sizes (Fig. 5), occurred when a maximum from tagging studies (Pezzack and Duggan, 1990). The intermolt period of 6 years was used in the simulation.

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