Changes in biological parameters of Atlantic sharpnose shark Rhizoprionodon terraenovae in the Gulf of Mexico: evidence for density-dependent growth and maturity?

Abstract

Biological parameters of the Atlantic sharpnose shark Rhizoprionodon terraenovae in the northern Gulf of Mexico were re-examined to test for potential changes due to density dependent responses. Biological data from published studies in the Gulf of Mexico collected during the period 1979–1984 were compared with data collected during the period 1998–2001. For combined sexes, the von Bertalanffy growth parameters were L∞ = 110.8 cm total length, K = 0.39 year–1 and to = –0.86 year during 1984 and L∞ = 94.0 cm total length, K = 0.73 year–1 and to = –0.88 year during 1998–2001. Median size for males and females at maturity decreased from approximately 78.3 and 80.2�cm total length (TL), respectively, in 1979–1980 to 72.6 and 75.8 cm TL, respectively, in 1998–2001. Age at maturity for combined sexes also decreased from 2.3 years in 1979–1980 to 1.4 years in 1998–2001. Litter size was similar between periods (5.0 embryos), as was the average size of embryos close to parturition (32 cm TL; 130–150�g). Growth rates using observed mean size-at-age data were higher from 1998–2001 for early ages (0–2.5 years). The observed decrease in maturity and increased growth rate lends support to the hypothesis of a compensatory response, although it could not be fully determined whether the response was due to differences in methodology among studies, anthropogenic influences or natural causes.

Life history of the cownose ray, Rhinoptera bonasus, in the northern Gulf of Mexico, with comments on geographic variability in life history traits

“…The lack of older individuals can greatly affect growth model parameter estimates (Cailliet & Goldman 2004). Although researchers have suggested that changes in life history parameters over time many be the result of compensatory changes in response to anthropogenic factors such as fishing (Walker & Hislop 1998, Carlson & Baremore 2003, we do not believe that is the case for the cownose ray as no directed fishery exists for this species. Finally, these results may truly reflect differences in regional growth patterns.…”

Do differences in life history exist for blacktip sharks, Carcharhinus limbatus, from the United States South Atlantic Bight and Eastern Gulf of Mexico?

“…Blacktip sharks have been heavily harvested in the Gulf of Mexico since the 1980's (NMFS 2003), thus the observed decrease in length-and age-atmaturity and increased growth rate lends support to the potential for a density-dependent compensatory response. Compensatory growth and reproductive responses have been documented in a few species of sharks (Sminkey and Musick 1995;Carlson and Baremore 2003). For reasons previously outlined, it could not be determined if these temporal changes in age and growth were due to differences in methodology, anthropogenic influences, or natural causes.…”

Demographic analyses of the dusky shark, Carcharhinus obscurus, in the Northwest Atlantic incorporating hooking mortality estimates and revised reproductive parameters

“…Sminkey and Musick (1995) found significant changes in length at age of several age classes of juvenile sandbar sharks, Carcharhinus plumbeus, following exploitation. Carlson and Baremore (2002) reported decreases in length and age at maturity in the Atlantic sharpnose shark, Rizoprionodon terraenovae, following exploitation. Cassof et al (2007) reported lower ages at maturity in the porbeagle, Lamna nasus, as a result of exploitation.…”