Size‐selective Harvesting Alters Life Histories of a Temperate Sex‐changing Fish

Abstract

Selective mortality, whether caused naturally by predation or through the influence of harvest practices, initiates changes within populations when individuals possessing certain heritable traits have increased fitness. Theory predicts that increased mortality rates will select for changes in a number of different life history characteristics. For example, fishing often targets larger individuals and has been shown repeatedly to alter population size structure and growth rates, and the timing of maturation. For sex-changing species, selective fishing practices can affect additional traits such as the mature population sex ratio and the timing of sexual transformation. Using historical comparisons, we examined the effects of exploitation on life history characteristics of California sheephead, Semicossyphus pulcher, a temperate protogynous (female-male sex changer) labrid that inhabits nearshore rocky environments from central California, USA, to southern Baja California, Mexico. Recreational fishing intensified and an unregulated commercial live-fish fishery developed rapidly in southern California between the historical and current studies. Collections of S. pulcher from three locations (Bahia Tortugas, Catalina Island, and San Nicolas Island) in 1998 were compared with data collected 20-30 years previously to ascertain fishery-induced changes in life history traits. At Bahia Tortugas, where fishing by the artisanal community remained light and annual survivorship stayed high, we observed no changes in size structure or shifts in the timing of maturation or the timing of sex change. In contrast, where recreational (Catalina) and commercial (San Nicolas) fishing intensified and annual survivorship correspondingly declined, males and females shifted significantly to smaller body sizes, females matured earlier and changed sex into males at both smaller sizes and younger ages and appeared to have a reduced maximum lifespan. Mature sex ratios (female:male) increased at San Nicolas, despite a twofold reduction in the mean time spent as a mature female. Proper fisheries management requires measures to prevent sex ratio skew, sperm limitation, and reproductive failure because populations of sequential hermaphrodites are more sensitive to size-selective harvest than separate-sex species. This is especially true for S. pulcher, where different segments of the fishery (commercial vs. recreational) selectively target distinct sizes and therefore sexes in different locations.

Drivers of protogynous sex change differ across spatial scales

“…This argument generally assumes the length and age of sexual transition is fixed with the corollary that gonochoristic species have a greater capacity to respond to fishing pressure than protogynous species [12,13]. However, both experimental and observational studies have demonstrated a high adaptive capacity, especially in the labrids, to alter the timing of sex change in response to selective removal of individuals [7,14,15]. Although, this response may differ across species [9] and its use as a compensatory mechanism when faced with heavy fishing pressure will be dependent on the intensity and extent of extraction on the population.…”

Reproductive pattern of an exploited dusky grouper <i>Epinephelus marginatus</i> (Lowe 1834) (Pisces: Serranidae) population in the western Mediterranean

“…The timing and duration of spawning, size/age at maturation and fecundity can change in response to environmental factors and to growth and mortality rates of the population (Stearns and Crandall, 1984;Wootton, 1990;Sanchez-Lizaso et al, 2000;Gust, 2004;Hamilton et al, 2007). In hermaphrodite species, the mating system and sex inversion are also considered to be adaptable and are probably the result of social interaction rather than genetic cues (Warner and Swearer, 1991;Mackie, 2003;Muñoz and Warner, 2003a;Munday et al, 2006).…”

Effects of stress on sex change in Crepidula cf. marginalis (Gastropoda: Calyptraeidae)

“…To become female earlier is advantageous if life expectancy is short and the life-time reproductive output threatened by high mortality risk. Changes in mortality rates in the field due to fisheries have resulted in observed downward shifts in the size at sex change in both fish and shrimp (e.g., Charnov, 1981;Charnov, and Hannah, 2002;Hamilton et al, 2007;Kazancioğlu and Alonzo, 2010). It seems likely that these downward shifts in size at sex change are the result of animals adjusting to altered social environments but other scenarios are possible.…”