Compensatory Growth of the Sandbar Shark in the Western North Atlantic Including the Gulf of Mexico

Abstract

The number of Sandbar Sharks Carcharhinus plumbeus in the western North Atlantic Ocean has experienced a drastic decline since the early 1980s, reaching a minimum during the early 1990s. Catch rates in the early 1990s were a mere 25% of those during the 1980s. According to several fishery‐independent surveys, the low point in Sandbar Shark abundance followed a period of high exploitation. Growth models fit to age–length data collected from 1980 to 1983 and from 2001 to 2004 were compared to investigate potential changes in parameter estimates that might reveal compensatory responses in the Sandbar Shark population. Statistical differences were found between the model parameters for the two time periods, but the differences in growth rates were minimal. The parameters from the three‐parameter von Bertalanffy growth model for female sharks during the 1980–1983 and 2000–2004 time periods were as follows: L∞ = 188.4 and 178.3 cm FL; k = 0.084 and 0.106; and t 0 = −4.097 and −3.41. For males the growth parameters were as follows: L∞ = 164.63 and 173.66 cm; k = 0.11 and 0.11; and t 0 = −3.62 and −3.33. The estimated age at 50% maturity for female Sandbar Sharks changed from 15 years to 12.49 years between the two time periods. Received December 21, 2012; accepted March 28, 2013

Grow fast, die young: Does compensatory growth reduce survival of juvenile blacktip sharks (Carcharhinus limbatus) in the western Gulf of Mexico?

“…Despite missing peak prey abundance, late parturition cohorts exhibited accelerated first year growth rates, suggesting that food resources are not limiting for YOY blacktips and other predators in Texas estuaries. Compensatory growth previously documented in other shark populations resulted from declines in abundance, with subsequent density‐dependence release promoting increased growth rates (e.g., Carlson & Baremore, 2003 ; Romine et al, 2013 ). Comparatively, first year growth among blacktips in Texas was not influenced by shark abundance, and reductions in prey populations led to increased first year growth rates, supporting our hypothesis that food is not limiting.…”

“…In response to fishing pressure, juvenile Atlantic sharpnose sharks ( Rhizoprionodon terraenovae ) exhibited density‐dependent compensatory growth in the Gulf of Mexico (GOM), which led to earlier sexual maturation (Carlson & Baremore, 2003 ). Sandbar sharks ( Carcharhinus plumbeus ) exhibited a similar compensatory response to fishing pressure in the northwestern (NW) Atlantic (Romine et al, 2013 ; Sminkey & Musick, 1995 ), and NW Atlantic porbeagles ( Lamna nasus ) exhibited faster growth rates in response to fisheries harvesting (Cassoff et al, 2007 ). However, the compensatory response of porbeagles varied across age‐classes, with subadults growing faster and juveniles growing slower under exploitation (Cassoff et al, 2007 ).…”

Using oxytetracycline validation for confirmation of changes in vertebral band-pair deposition rates with ontogeny in sandbar sharks (Carcharhinus plumbeus) in the western North Atlantic Ocean

“…Discrepancies exist among the previously published age and growth studies on sandbar sharks, largely because of lack of validation of band-pair periodicity in the early vertebral studies (Casey et al, 1985;Casey and Natanson, 1992;Sminkey and Musick, 1995;Andrews et al, 2011;Hale and Baremore, 2013;Romine et al, 2013). Both growth curves generated in models from tag-recapture data and results from bomb radiocarbon analyses suggest slower growth and a longer lifespan than that estimated by using band-pair counts on vertebral centra (Casey and Natanson, 1992;Andrews et al, 2011).…”