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RESEARCH ARTICLE

Does proximity to freshwater refuge affect the size structure of an estuarine predator (Carcharhinus leucas) in the north-western Gulf of Mexico?

Philip Matich https://orcid.org/0000-0003-4327-7109 A B I , Robert J. Nowicki C , Jonathan Davis H , John A. Mohan A , Jeffrey D. Plumlee D , Bradley A. Strickland E , Thomas C. TinHan A , R. J. David Wells A F and Mark Fisher G
+ Author Affiliations
- Author Affiliations

A Department of Marine Biology, Texas A&M University at Galveston, 1001 Texas Clipper Road, Galveston, TX 77553, USA.

B Sam Houston State University, Texas Research Institute for Environmental Studies, 2424 Sam Houston Avenue Suite B-8, Huntsville, TX 77341, USA.

C Elizabeth Moore International Center for Coral Reef Research and Restoration, Mote Marine Laboratory, 24244 Overseas Highway, Summerland Key, FL 33042, USA.

D Institute of Marine Sciences, University of North Carolina at Chapel Hill, 3431 Arendell Street, Morehead City, NC 28557, USA.

E Marine Sciences Program, Department of Biological Sciences, Florida International University, 3000 NE 151st Street, North Miami, FL 33181, USA.

F Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843, USA.

G Texas Parks and Wildlife Department, Coastal Fisheries Division, Rockport Marine Science Laboratory, 702 Navigation Circle, Rockport, TX 78382, USA.

H Present address: 4001 Beverly Avenue, Orange, TX 77632, USA.

I Corresponding author. Email: pmati001@fiu.edu

Marine and Freshwater Research 71(11) 1501-1516 https://doi.org/10.1071/MF19346
Submitted: 3 November 2019  Accepted: 2 April 2020   Published: 21 May 2020

Abstract

The life histories of estuarine species are often adapted to the environmental variability they experience. However, estuaries are increasingly vulnerable to natural and anthropogenic changes, necessitating an understanding of how shifting conditions affect the survival, behaviour and population structure of estuarine-dependent animals. In this study we used data from fisheries-independent surveys collected across six estuaries with variable salinity regimes in Texas, USA, from 1975 to 2016 to investigate the role sources of freshwater inflow play in shaping juvenile bull shark Carcharhinus leucas size structure. High frequencies of co-occurrence with similarly sized conspecifics (59% of capture events) suggest bull sharks segregated within Texan estuaries based on body size. Bull shark sizes increased with distance to the nearest source of freshwater inflow, although effect sizes were small and access to freshwater habitats may be more important in shaping size-dependent distribution patterns. River mouths were disproportionately used by smaller juveniles (<90-cm total length, TL) and avoided by larger juveniles (>135 cm TL). However, the use of river mouths decreased in estuaries characterised by limited freshwater inflow and greater variability in salinities at river mouths, highlighting geographic differences in the functions these habitats provide as potential environmental and predator refugia. Young-of-the-year (i.e. age-0) sharks also increased their use of river mouths throughout the 40-year study period, revealing the growing importance of river mouths as potential nursery habitats.

Additional keywords: bull shark, estuary, euryhaline, nursery, ontogenetic shift.


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