The impact of intrapopulation variability in reproductive traits on population reproductive potential of Grand Bank American plaice (Hippoglossoides platessoides) and yellowtail flounder (Limanda ferruginea)

Abstract

In geographically extensive fish populations the potential exists for reproductive traits to vary over the population’s range but, the impact that such intrapopulation variability has on overall population reproductive potential has not been formally assessed. Here intrapopulation spatial variability in size at maturity and fecundity are demonstrated for Grand Bank American plaice (Hippoglossoides platessoides) and yellowtail flounder (Limanda ferruginea). Recognition of intrapopulation variability in these reproductive traits coupled with spatial variability in abundance resulted in an increase in estimated population total egg production (TEP) by as much as 10¹⁴ eggs for American plaice and 10¹⁵ eggs for yellowtail flounder as compared to assessment of TEP for the population as a whole. Results highlight the need to explore variability in life history traits not only between but, also within populations and emphasizes the need for sufficient spatial coverage during sampling in order to assess the reproductive potential of fish populations.

Introduction

The nature of perceived relationships between animals and their environment can depend on the scale at which the system is viewed (Levin, 1992). The scale of observation, however, is usually set a priori and can lead to important aspects of the system being overlooked (Peterson and Parker, 1998). In fishes, for example, spatial variability in life history traits is often assessed at the population level, with less attention directed at potential spatial variability within populations. Even less emphasis has been placed on understanding the impact that potential intrapopulation variability in life history traits might have on population productivity.

Reproduction in fishes is heavily dependent on environmental conditions. For example, conditions that support increased growth rate and higher fish condition, promote maturation at an earlier age/size (Gadgil and Bossert, 1970, Rijnsdorp, 1993, Morgan and Colbourne, 1999, Olsen et al., 2004). In addition, fish of larger size and better condition produce more eggs than smaller and poorer conditioned individuals (Gundersen et al., 1999, Kraus et al., 2002, Blanchard et al., 2003, Lambert et al., 2006) and may have a higher probability of successfully spawning (Rideout et al., 2005). Temperature can also influence fish reproduction (Hodder, 1965, Kjesbu et al., 1998, Yoneda and Wright, 2005a, Yoneda and Wright, 2005b), not only through its impact on growth rate, but also by altering the animal's energy budget or by directly affecting the biochemical processes within developing gametes.

Because environmental conditions can be highly variable over space and time, scale of observation is an important consideration in studying the reproductive and recruitment processes in fishes (Schneider, 2001). Size at maturity and fecundity at length data are typically presented as population values with little consideration for intrapopulation variability. Only a limited number of studies on fishes have reported maturation to be heterogeneous across the range of a population (Korsbrekke, 1999, Bromley, 2000, Gerritsen et al., 2003, Armstrong et al., 2004, Boulcott et al., 2007). Even fewer have demonstrated differences in fecundity between areas within a population (Pinhorn, 1984, Rijnsdorp, 1991, Kjesbu et al., 1998, Nash et al., 2000). It has been suggested that such regional differences might have significant consequences for resource management (Boulcott et al., 2007) but the impact that failure to recognize this variability has on perception of stock reproductive potential has rarely been examined.

American plaice (Hippoglossoides platessoides) and yellowtail flounder (Limanda ferruginea) are two commercially important flatfish species with overlapping distributions off the east coast of Canada. For each species, individuals over the entire Grand Bank, a large geographical area encompassing three Northwest Atlantic Fisheries Organization (NAFO) Divisions (3LNO), are treated as a single population (Fig. 1). Temporal variability in both maturity (Morgan and Colbourne, 1999, Walsh and Morgan, 1999) and fecundity (Rideout and Morgan, 2007) have been reported for the Grand Bank populations of both species. Large scale spatial variability in these reproductive traits has also been demonstrated, with differences between these and other nearby populations (Rideout and Morgan, 2007). The large geographic area covered by the Grand Bank populations of flatfish suggests the potential for environmental conditions to vary over the range of the populations, and hence create the potential for reproductive traits to vary over a smaller spatial scale within the populations. Intrapopulation spatial variability in reproductive traits has been reported for Grand Bank American plaice based on limited sampling (Pitt, 1964, Pitt, 1966) but has not been examined for yellowtail flounder. Here we examine not only intrapopulation variability in maturation and fecundity for both American plaice and yellowtail flounder, but also investigate the potential impacts that failure to recognize intrapopulation variability in life history traits has on the perceived reproductive potential of these fish populations.