Size-at-maturity shift in a male-only fishery: factors affecting molt-type outcomes in Newfoundland and Labrador snow crab (Chionoecetes opilio)

Abstract

A sex-asymmetric downward shift in size-at-terminal-molt has recently occurred in males in some portions of the Newfoundland and Labrador (NL) snow crab stock range, a first known occurrence for such processes in snow crab (Chionoecetes opilio) stocks. This study examines plausible factors promoting the shift in size-at-terminal-molt [synonymous with size-at-maturity (SaM)] including individual size, temperature, population density, and sex ratio. Analyses highlight expanse of cold water and large male density as being significant predictors of molt-type outcomes. A confluence of cold conditions and low density of large males promoted the SaM shift. In turn, the low male density was associated with recently elevated fishery exploitation rates under quota-controlled management. It remains unknown the extent to which the reduction in terminal size reflects a phenotypic vs. genotypic process. Factors affecting skip-molting in male snow crab are investigated, and we find that skip-molting occurs most frequently under extreme cold and high population density conditions. Potential complications arising from altered growth dynamics are discussed. Overall, the results advance knowledge on intraspecific competition processes within snow crab populations and inform fisheries management systems that male-only harvest strategies do not provide full protection from biological harm to aquatic resources through fishing.

Older but not wider. Impacts of molt frequency variation on age and growth dynamics of Newfoundland & Labrador snow crab (Chionoecetes opilio) toward corroborating gastric mill 2 age estimations

“…Importantly, high population densities of large males appear to promote skip-molting over terminal-molting for adolescent males not undertaking a regular molt, as shown in Mullowney and Baker (2021) and inferred from previous studies (Hébert et al, 2002;. This outcome promotes overall greater sizes within populations through an overall greater number of molts occurring over ontogeny as the "stop point" for molting is prolonged.…”

“…At-present, interactions and synergies between temperature and population density (of intermediate-large size males in particular) in regulating molt frequency and terminal sizes within populations are not well understood. However, the recent observations by Mullowney and Baker (2021) suggest density of large males could override temperature in regulating growth. For example, over the past 25 years larger size-atterminal molt has occurred in males in cold water dense populations (Divisions 3LNO, Figure 1) than in warmer water less dense populations (Divisions 2J3K, Figure 1) within the NL stock range.…”

“…In populations where this occurs, it can lead to average molt frequency rates beyond about instars VII-VIII becoming less than annual. In wild NL populations, Mullowney and Baker (2021) recently showed skip-molting to be highest in 50-100mm CW adolescent males, with rates as high as 25-50% of individuals skip-molting in some populations in some years. These sizes roughly conform to instars IX -XII (Figure 1).…”

“…Coincidental increasing probabilities of skip-molting (near-exclusive to males) and terminally molting (applicable to both sexes) with individual size after mid-life stages (instars) are reached slows molt frequency over ontogeny. For example, to focus on a relatively small CW range of 20mm centered near legal-size of 95mm CW, Mullowney and Baker (2021) showed that over the past twenty-five years males throughout different portions of the NL stock range had a 25-40% probability of being terminally-molted at 80mm CW versus a 50-75% probability of being terminally-molted at 100mm CW. These sizes most closely conform to instars XI and XII (Figure 1) and demonstrate the pronounced fisheries yield losses that can be associated with losses to terminal molt at small sizes.…”

“…The effects of population density on molt frequency and terminal size have not been as thoroughly studied as those of temperature, but knowledge on the importance of population density in regulating growth dynamics is increasing. Low density of largest (most competitive) males has recently been linked to small terminal molt sizes in males, theoretically related to earlier allocation of energy into reproduction over growth following relaxed intraspecific competition for access to food and space or other resources (Mullowney and Baker, 2021). Subsequently, Murphy (2021) showed a linkage of terminal molt size in males to recruitment density, with individuals in strong cohorts reaching overall larger terminal sizes than those in weak cohorts.…”