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

Abstract

Effective conservation and management necessitate an understanding of the ecological mechanisms that shape species life histories in order to predict how variability in natural and anthropogenic impacts will alter growth rates, recruitment, and survival. Among these mechanisms, the interaction between parturition timing and prey availability frequently influences offspring success, particularly when postnatal care is absent. Here, we assess how parturition timing and nursery conditions, including prey abundance and environmental conditions, influence the growth and potential survival of blacktip sharks (Carcharhinus limbatus) in western Gulf of Mexico (GOM) estuaries over their first year. Catch data from long‐term gillnet monitoring allowed for clear delineation of cohorts based on size frequency distribution plots, and showed that late parturition cohorts born in estuaries with fewer prey resources exhibited more rapid growth than early parturition cohorts that experienced more abundant prey. Compensatory behaviors that promoted accelerated growth led to reduced second year residency, likely due to reduced survival resultant from greater risk taking and potentially due to reduced site fidelity attributed to larger body size. Water temperatures influenced blacktip growth rates through physiological increases in metabolism and potential premigratory foraging cues associated with cooling temperatures. Gradual warming of the GOM (0.03°C year−1) was also correlated with earlier parturition across the study period (1982–2017), similar to other migratory species. Considering current trends in climate and associated phenological shifts in many animals, testing hypotheses assessing compensatory growth‐risk trade‐offs is important moving forward to predict changes in life histories and associated recruitment in concert with current and future conservation actions, like wildlife management.

Delineation of blacktip shark (Carcharhinus limbatus) nursery habitats in the north‐western Gulf of Mexico

“…At the ecosystem‐level, only the Matagorda estuary was delineated as an age 0 nursery. This was not unexpected based on the high abundances of blacktips described within this ecosystem by previous studies (Froeschke, Stunz, & Wildhaber, 2010; Matich, Plumlee, & Fisher, 2021; Plumlee et al , 2018), and its moderate freshwater inflow and thus moderate‐high salinities throughout most habitats (Matich et al , 2020). However, temporal variability in blacktip CPUE within and adjacent to this estuary was apparent.…”

“…Yet, it is unclear what caused temporal variability in blacktip abundance, which was evident across all estuaries in the study system and has been described coastwide (Plumlee et al , 2018). Our previous work indicates that parturition timing, environmental conditions and food availability play an important role in first‐year survival and thus abundances of age 0 blacktips in the study area facilitated though behavioural compensatory mechanisms attributed to growth‐risk trade‐offs (Matich, Plumlee, & Fisher, 2021). Our results here suggest that greater environmental variability could exacerbate trade‐offs faced by blacktips, as well as other elasmobranchs that also exhibit periodicity in abundance within Texas estuaries ( Hypanus sabinus , R. terraenovae ; Plumlee et al , 2018), consequently altering elasmobranch communities within north‐western Gulf of Mexico estuaries on annual‐decadal time scales (Bethea et al , 2015; Castro, 1993; McCandless et al , 2007).…”

“…Large sharks, including adult blacktips, pose the threat of predation to smaller sharks in continental shelf waters (see references within Cottrant et al , 2021; Matich, Plumlee, Weideli, & Fisher, 2021). As such, while overwintering juvenile blacktips must use these risky habitats in the Gulf of Mexico in response to cooling temperatures (Heupel et al , 2004; Hueter et al , 2005; Matich, Plumlee, & Fisher, 2021), juveniles are found in shallow, nearshore waters during most of the year, which are expected to provide refuge during early periods of development ( e.g ., McCandless et al , 2007) and may be used by blacktips for longer durations as water temperatures rise (Matich, Plumlee, & Fisher, 2021). However, risk is not the only factor that influences habitat use (Knip et al , 2010; Speed et al , 2010) nor contributes to ontogenetic shifts (Grubbs, 2010), and our results indicate that safety is not the primary factor that defines blacktip nursery habitats within north‐western Gulf of Mexico estuaries.…”

“…Blacktips reach reproductive maturity at ca . age 5–6 years (Baremore & Passerotti, 2012), prior to which juveniles are found in bays, sounds and estuaries, with excursions into waters over the continental shelf during winter due to thermoregulatory limitations (Heupel & Hueter, 2002; Hueter et al , 2005; Matich, Plumlee, & Fisher, 2021; McCandless et al , 2007). Nearshore hydrology is heterogeneous across the region, with spatiotemporal variability in freshwater and tidal flow attributed to differences in precipitation and evaporation, as well as human water management (US EPA, 1999).…”

“…Based on rarefaction analysis, only stations with eight or more sampling events during the study period were included in analyses. Blacktip age was based on our previous work within the study system (Matich, Plumlee, & Fisher, 2021): sharks <70.6 cm TL in spring and <91.6 cm TL in fall were classified as age 0.…”