Changes in Abundance and Certain Population Parameters of American Plaice on St. Pierre Bank off Newfoundland during 1972–1994, with Implications for Fisheries Management

Abstract

Changes in abundance and certain population parameters of American plaice Hippoglossoides platessoides in Northwest Atlantic Fisheries Organization (NAFO) subdivision 3Ps, south of Newfoundland, were examined over the period 1972–1994. During these years, there has been a substantial reduction in population size and in more recent years, there has been a redistribution of the remaining population to deeper, warmer water than previously observed. Despite the substantial changes in stock size and distribution, there is no clear evidence that growth patterns have changed nor has there been any truncation of the age structure of the population, both of which commonly occur if overexploitation takes place. On the other hand, age and size at 50% sexual maturity for both males and females declined over the same period. This stock has been managed through quota regulation since 1974; however, the quota rarely restricted the fishery. During the 1980s, when the greatest declines in stock abundance occurred, the commercial exploitation rate was far too low to have been an important contributor to the observed decrease in population size, It is concluded that management measures during this period were ineffective in maintaining stock size at the target level because the stock decline was not the result of fishing. The reasons for such substantial reductions in the stock remain unknown. During the 1990s, however, as this and other groundfish stocks were declining rapidly, a substantial increase in fishing effort was directed towards this stock in an attempt to fully utilize existing quotas. Annual catches thus were maintained at a moderate level despite a rapidly declining population, producing exploitation rates much higher than those of the 1980s. By 1995, the stock was estimated to be so reduced that a total fishing moratorium was introduced for American plaice in NAFO subdivision 3Ps. The result of this measure is yet to be evaluated.

Sex-specific temperature distribution in four populations of American plaice Hippoglossoides platessoides

“…Fisheries can be a potent source of sizeselective mortality, and fishing pressure appears to have varied substantially between the populations in our study. Exploitation appears to have been lightest for the NE Newfoundland population and heaviest for the Grand Bank population (Bowering et al 1996. The extent of size selection by a fishery will depend on growth rates and on selectivity of the fishery as well as on the overall level of fishing mortality.…”

“…Bottom temperatures over the Newfoundland Shelf were 0.25 to 1.0°C below normal from 1986 to 1994, the longest continuous period of negative temperature anomalies since measurements began in 1946 (Colbourne et al 1997). The shifts in the temperature distribution of plaice involve shifts to deeper waters (Bowering et al 1996. One hypothesis is that these shifts are a response to the cold conditions of the late 1980s and early 1990s, with plaice moving into deeper water to avoid the unusually cold water on the shelves and banks.…”

Maturation of Newfoundland American plaice (Hippoglossoides platessoides): long-term trends in maturation reaction norms despite low fishing mortality?

“…It is found throughout the northwest Atlantic (Bowering et al, 1996). There are three populations of American plaice living (i) off the east and south coasts of Newfoundland, Labrador, and Northeast Newfoundland (Northwest Atlantic Fisheries Organization, NAFO, Divisions 2J3K), (ii) on the Grand Bank (NAFO Divisions 3LNO), and (iii) on St Pierre Bank (NAFO Division 3Ps).…”

“…Although there are seasonal movements of plaice in the study area (Morgan and Brodie, 1991), tagging indicates that the fish do not undertake long-distance movements (Morgan, 1996). In particular, the three populations investigated here are thought to be distinct, with little or no movement between them (Bowering et al, 1996;Bowering et al, 1998).…”

“…Here we apply this method to three populations of Newfoundland American plaice. These populations have all experienced large declines in abundance, but have been subject to different levels of fishing mortality (Bowering et al, 1996;Morgan et al, 2002a, b). The estimation of maturation reaction norms for these stocks offers a unique scientific opportunity to examine possible evolutionary change in the maturation processes of populations of the same species that have experienced different levels of exploitation.…”