Disentangling the effects of size-selective mortality, density, and temperature on length-at-age

Abstract

The relative importance of size-selective mortality, density-dependent growth, and temperature on growth of a commercial fish population was investigated using an integrated statistical analysis. Two indices of size-selective mortality were determined using otolith backcalculations. One index measured the direct effect on population mean growth increments in the year of the growth increment. The second index measured the cumulative effect on the growth potential of a cohort. Indices of population density, occupied temperature, and bottom temperature were developed from annual synoptic research vessel surveys of the population. We simultaneously tested effects of these factors using a modified von Bertalanffy growth model. The strongest effect was variation in size-selective mortality, followed by a negative effect of population density and a weak positive effect of occupied temperature. Effects of bottom temperature conditions were not significant. Failure to simultaneously consider alternative mechanisms, especially size-selective mortality, can lead to incorrect conclusions about the role of environmental factors in determining growth of fishes.

Age and growth of the dog snapper Lutjanus jocu (Bloch & Schneider, 1801) in Abrolhos Bank, Northeastern Brazil

“…The estimated growth rate would be overestimated due to the selective mortality of the fast growing individuals (Sainsbury, 1980). Another source of bias may be the survival rate; the smaller fish would have higher survival rates under selective mortality than the average population (Sinclair et al, 2002). The fishing gears used to catch dog snappers in Abrolhos Bank are highly selective; most individuals captured are between 35 and 65 cm FL.…”

Trait changes in a harvested population are driven by a dynamic tug-of-war between natural and harvest selection

“…Finally, selection differentials provided in the reproductive investment analysis and in Fig. 2 were estimated for each year and each age as the mean back-calculated length of females that escaped the fishery in that year (but were caught eventually) minus the mean length of females of the same age caught into the fishery in that year (9) for the entire study period .…”

“…Accordingly, recent studies in nature indicate a correlation between increased harvest pressure and phenotypic changes in plants (5) and vertebrates (6). Harvest-induced phenotypic changes have been studied extensively in fisheries, where harvest is often associated with decreased somatic growth and/or decreased age and size at maturity (7)(8)(9). Under laboratory conditions, artificial selection against large size has been shown to induce rapid evolution toward slower growth (10) and to promote genetically based reductions in fecundity, larval viability, and foraging efficiency (11).…”

Population Changes and Location-Specific Differences for Otolith-Derived Age and Growth of Recreationally Harvested Spotted Seatrout (Cynoscion nebulosus) From Alabama in 2007

“…Because metabolism has a significant role in otolith growth (Johnson et al, 2002;Hussy and Mosegaard, 2004), environmental variations that impact metabolic processes can also be quantified through changes in otolith morphometrics (Sinclair et al, 2002;Bortone et al, 2006). Otolith-based data of age and growth can identify changes within populations by quantifying cohort strength (Wilson et al, 2001), age related bottlenecks (Sinclair et al, 2002), and the impacts of habitat on growth (Rooker et al, 1999;Allman, 2007). Otolith-based age and growth measurements for spotted seatrout (Cynoscion nebulosus) have been positively correlated with seagrass abundance and salinity levels (Bortone et al, 2006).…”