Demographics influence reproductive output in queen conch (Lobatus gigas): implications for fishery management

Abstract

The queen conch (Lobatus gigas) is harvested intensively throughout most of the tropical western Atlantic for its meat, shell, and pearls. If sustainable harvest is to be achieved, fishery managers will have to incorporate into management strategies the species’ biology and demography. However, no long-term information exists that links reproductive behavior with age structure in queen conch. As such, we examined queen conch demographics across large spatial and temporal scales in the Florida Keys, where queen conch fishing has been banned since 1986. Our results corroborate earlier reports showing density-dependent reproductive behavior. We never observed mating when aggregation densities fell below 204 adults ha−1; spawning was not observed at aggregation densities less than 90 adults ha−1. Evaluating reproductive behavior by age class, based on lip thickness (i.e., young adult, >10–15 mm; adult, >15–25 mm; very old adult, >25 mm), showed that young adults mated and spawned less often than adults and very old adults. However, the number of eggs in an egg mass showed no significant association with lip thickness. So, while age did not seem to affect the number of eggs in an egg mass, young adults as a group would have lower reproductive output because they do not engage in reproductive activities as often as older queen conch, probably because some of the younger, thinner-lipped conch were not yet sexually mature. Our results indicate that the maintenance of age structure and density is crucial to the stability of queen conch populations and thus sustainable harvest. Therefore, the soundest management approaches are those that maximize reproductive output from aggregations of mature queen conch whose densities exceed minimum thresholds to avoid depensatory effects.

Size at Maturation, Spawning Variability and Fecundity in the Queen Conch, Aliger gigas

“…The enclosures and stocking densities were not designed to mimic natural conditions (other than to be located on a natural spawning ground), and this could have affected observed results. However, the low-density treatment, at 143 conch/ ha, was well within the range of densities within spawning sites reported by Stoner and Ray-Culp (2000), Stoner et al (2012a) and Delgado and Glazer (2020). Indeed, this density was lower than the mean density reported by Stoner and Ray-Culp (2000; 209 conch/ha) and Delgado and Glazer (2020; 610 conch/ha).…”

“…However, the low-density treatment, at 143 conch/ ha, was well within the range of densities within spawning sites reported by Stoner and Ray-Culp (2000), Stoner et al (2012a) and Delgado and Glazer (2020). Indeed, this density was lower than the mean density reported by Stoner and Ray-Culp (2000; 209 conch/ha) and Delgado and Glazer (2020; 610 conch/ha). Although the high-density treatment, at 2,000 conch/ha, is well above average reported values, it still is less than the maxima reported by Stoner and Ray-Culp (2000) and Delgado and Glazer (2020), at 2,293 and 3,133 conch/ha, respectively.…”

“…Indeed, this density was lower than the mean density reported by Stoner and Ray-Culp (2000; 209 conch/ha) and Delgado and Glazer (2020; 610 conch/ha). Although the high-density treatment, at 2,000 conch/ha, is well above average reported values, it still is less than the maxima reported by Stoner and Ray-Culp (2000) and Delgado and Glazer (2020), at 2,293 and 3,133 conch/ha, respectively. In these latter studies, a high proportion of conch was engaged in reproductive activity (spawning, copulating, pairing) at these extreme densities.…”

“…Theory argues that mating (copulation) rate will increase with increasing density, but if females have control over mating (i.e., no male harassment) and partners are chosen at random, mating rates can remain constant across different densities (Sprenger et al 2011). At present, little is known about the mating systems governing queen conch reproduction, although the proportion of the population engaging in reproduction increases with increasing density up to some plateau (Stoner and Ray-Culp 2000, Stoner et al 2012, Delgado and Glazer 2020. That there may be tensions between males and females and competition among males is suggested by the observations of a high proportion of copulations occurring during egg deposition (up to 99% in Strombus pugilis) when the females remain stationary, of females dragging a copulating male with them when not spawning, and of multiple males attempting to copulate with a single female (Reed 1992(Reed , 1995.…”

“…This finding could be indicative of either sexual facilitation or sperm limitation, but there was no obvious indication of the latter affecting the females in the low-density treatment where there was only one male. If sexual facilitation occurs, and increased contact with males stimulates gametogenesis and egg production (Crews et al 1986), this would be still another reason to protect areas with high densities of spawning queen conch adults (Delgado and Glazer 2020).…”