Estuarine nursery areas provide a low-mortality environment for young bull sharks Carcharhinus leucas

Abstract

Defining the role and impact of mortality within aquatic populations is often difficult. As a result, we still lack a clear understanding of the level of mortality in natural populations and the role habitat may play in survival. We used long-term acoustic monitoring of neonate bull sharks Carcharhinus leucas in a south Florida estuary to determine mortality rates within this population. Estimates of natural, fishing and total mortality varied among the years examined, but not significantly, suggesting similar survival among years with the majority of individuals (77%) surviving beyond 18 mo. Compared with other juvenile sharks, mortality rates of C. leucas were low. The results indicate that the use of mesohaline estuarine habitats by C. leucas may provide benefits that are not present in more marine habitats where their counterparts reside. Use of estuarine regions may be a successful ecological strategy for improving survival because of reduced predation and competition.

Mortality rates for two shark species occupying a shared coastal environment

“…Most direct estimates of mortality that have been provided for shark populations are based on neonate and juvenile individuals (e.g., Xiao et al, 1999;Gruber et al, 2001;Heupel and Simpfendorfer, 2002;Driggers et al, 2008;Heupel and Simpfendorfer, 2011). Studies have estimated mortality rates for juveniles because of the greater threat mortality poses to young sharks (Kinney and Simpfendorfer, 2008).…”

More analytical bite in estimating targets for shark harvest

“…Method 6 is used for populations with a stable age distribution, known rates of increase and an annual birth pulse so that distinct cohorts are recognisable as is the case in some larger shark species (e.g. McAuley et al 2007, Heupel & Simpfendorfer 2011. Here, the proportion of individuals surviving at the start of Age x, l x , is: l x = n x e rx /n 0 , where n x = the number observed in age class x to x +1 and n 0 = the number of births estimated from the fertility data.…”

Diversity in young shark habitats provides the potential for portfolio effects

“…Small sharks are both predators and prey, and how a species balances the trade-off between predator avoidance and energy uptake can often be related to its life history (Branstetter 1990), as well as competitive interactions between individuals (Brown 1999, Heithaus 2004. It is widely assumed that nursery areas benefit young sharks by providing ample food and protection from predators (Springer 1967, Branstetter 1990, Ryer et al 2010, Heupel & Simpfendorfer 2011). There are some notable exceptions to this longstanding assumption, including indications that these 2 factors may be mutually exclusive in at least some cases (Heithaus 2007, Heupel et al 2007).…”

“…Effective management of shark populations requires a detailed understanding of essential habitats and of how these are utilised by sharks. Accordingly, the location and functioning of shark nurseries has been a focus of recent research (Heupel et al 2007, McCandless et al 2007a, Heupel & Simpfendorfer 2011. Appropriate management of inshore shark nurseries is particularly important because they can be utilised by numerous shark species (Simpfendorfer & Milward 1993) and are in creasingly susceptible to a range of anthropogenic impacts and environmental change (Chin et al 2010, Knip et al 2010, Koehn et al 2011).…”