BRIEF RESEARCH REPORT article
Front. Mar. Sci., 17 November 2021 | https://doi.org/10.3389/fmars.2021.754686
Salinity and Water Temperature as Predictors of Bottlenose Dolphin (Tursiops truncatus) Encounter Rates in Upper Galveston Bay, Texas
Vanessa J. Mintzer1,2,3*† and Kristi L. Fazioli4†
1Galveston Bay Foundation, Kemah, TX, United States
2Fisheries and Aquatic Sciences Program, School of Forest, Fisheries, and Geomatics Sciences, University of Florida, Gainesville, FL, United States 3Wildlife Research Partnerships, Asheville, NC, United States
4Environmental Institute of Houston, University of Houston – Clear Lake, Houston, TX, United States
Bottlenose dolphins (Tursiops truncatus) that inhabit urban estuaries like Galveston Bay, Texas, are exposed to cumulative stressors including pollution, fisheries, shipping, freshwater inflows, and construction operations. With continuing development, it is imperative to understand the key environmental variables that make the Galveston Bay estuary suitable habitat for this protected species. The Galveston Bay Dolphin Research Program conducted monthly photo identification surveys of bottlenose dolphins in a previously understudied 186 km2 area in upper Galveston Bay (UGB). To understand occurrence patterns in this region, we calculated monthly encounter rates of dolphins (dolphins/km) for four consecutive years (2016–2019). Using multiple linear regression models, we investigated the relationship between encounter rates, and water temperature and salinity. Monthly encounter rates ranged from 0.00 to 1.23 dolphins/km with an average of 0.34 dolphins/km (SE = 0.05). Over 80% of the variance was explained by the predictor variables water temperature and salinity (R2 = 0.820). Water temperature had a positive linear effect on encounter rates at over 23.37°C (SE = 1.42). Accordingly, higher encounter rates occurred during months with warm temperatures (May–September) compared to cooler months (November–April), indicating a predictable yearly movement pattern. Moreover, salinity was a highly significant predictor variable, with encounter rates dropping linearly with decreases in salinity. Higher numbers of dolphins are found in UGB during summer, but an exodus of dolphins occurs with low salinity levels, regardless of the time of year and water temperature. These findings should be considered during infrastructure projects (i.e., flood gate system) that may alter dolphin habitat and prey availability.
snip...
Discussion Our findings indicate that bottlenose dolphins can be found in UGB year-round, but most leave during the cooler months. Annually, encounter rates rise during months with the warmest water temperatures (>23°C). Peak encounter rates will typically occur June–September; however, during periods of low salinity, encounter rates will likely decrease regardless of water temperature. Concurrent high temperature and salinity represent optimal environment conditions for dolphin presence in UGB.
As endothermic animals, dolphins depend on blubber and internal metabolic processes to maintain a stable body temperature. During the study period, dolphins experienced water temperature ranging from 10 to 32°C. Coastal bottlenose dolphins generally tolerate this range with changes in integument thickness and whole body conductance (Meagher et al., 2008; Carmichael et al., 2012). However, temperature may be a limiting factor for smaller dolphins (i.e., juveniles, calves and their mothers) (Yeates and Houser, 2008; Carmichael et al., 2012). Exploratory analyses showed that mother/calf pairs can be found in the study area year-round, but the proportion of groups with calves was higher in the warm months (>23°C) compared to the cold months (<23°C). Peak calving season for dolphins in Texas coastal waters is in the spring (Urian et al., 1996; Fernandez and Hohn, 1998), coinciding with a time when fewer dolphins are present in UGB. Furthermore, sightings of early neonates in UGB are rare (GDRP, unpublished data). If mothers with neonates frequent the study area only during warm months due to their offspring’s metabolic constraints, this could, in part, explain the effect of temperature on encounter rates. The drivers of movements and habitat use of mother/calf groups and calving females should be studied.
Prey migration is likely an important underlining mechanism for the annual encounter rate patterns related to temperature fluctuations (Irvine et al., 1981; Scott et al., 1990; Wilson et al., 1997). In and near Sarasota, Florida, for example, dolphins are found inside bays year-round, but many shift their distribution toward the Gulf during cooler months (Scott et al., 1990). Mullet migration has been suggested as a primary factor for this change, as mullet migrate from inshore areas to the Gulf to spawn in the fall and return to the bays in the spring (Scott et al., 1990). A similar pattern occurs in Texas where mullet leave the bays, from October to January, to spawn 40–50 miles offshore (Boyd, 2011). For many Galveston Bay fish species, two migration patterns have been recorded: the migration of spawning adults leaving the Bay and the migration of postlarvae and juveniles entering the Bay (Bechtel and Copeland, 1970). Although the exact timing of these migrations varies by species, most correspond with seasonal temperature changes and many enter the Bay as the temperature increases (Bechtel and Copeland, 1970). Additionally, a commercial shrimp fishery operates within the estuary, with trawler activity increasing in UGB during warm months, following shrimp life cycle and migration patterns [TWPD, 2002; Houston Advanced Research Center (HARC), 2020]. The strong association of foraging dolphins with this fishery (Henningsen and Würsig, 1991; Fertl, 1994; Moreno, 2005; Piwetz, 2019) has the potential to affect dolphin movements. Accordingly, dolphins likely return to UGB with rising water temperatures due to a combination of factors related to food availability.
Our results suggest that the UGB study area does not encompass the entire home ranges of the observed dolphins. Many individuals known to frequent UGB have been sighted south of our study area, in lower Bay, during the cooler months of the year (GDRP, unpublished data). However, it is unknown if most remain in the lower Bay or if they leave the estuary to utilize nearby coastal waters, or travel to other estuaries. Travel between Texas bays has been documented in other studies for some individuals (Blaylock and Hoggard, 1994; Maze and Würsig, 1999; Lynn and Würsig, 2002; Ronje et al., 2020). Previous studies have identified changes in abundance aligning with annual temperature fluctuation in studies in Galveston and other Texas estuaries (Shane, 1980; Henningsen and Würsig, 1991; Fertl, 1994; Wilson et al., 1997; Ronje et al., 2020). As in UGB, dolphin abundance peaks during the summer in other northern Texas coastal locations and decreases with cooler temperatures (Fertl, 1994; Wilson et al., 1997; Ronje et al., 2020), while in central and south Texas the opposite pattern has been documented (Shane, 1980; McHugh, 1989). Telemetry studies will be required to map detailed range patterns, but future winter observational studies focused on lower Galveston Bay could help determine if most UGB dolphins remain in the estuary during the cooler months and could identify calving hotspots.
During the study period, most dolphins left the study area during low salinity events (i.e., when salinity dropped below 8–11 ppt) and returned once salinity level had increased. This trend was evident in June 2016 (“Tax Day Flood”), September 2017 (Hurricane Harvey), and September 2019, when heavy precipitation led to salinity levels below 3.5 ppt and encounter rates dropped well below expected levels for the time of year. 2018 was the only year covered in this study with no major low salinity event, and it had the highest annual encounter rate of 0.44 d/km. On the other hand, 2019, a year with an El Niño event, had an exceptionally wet spring and early summer (TWDB, 2021) likely explaining the relatively lower encounter rates during these months and the annual average of only 0.28 d/km. Although more research is needed to understand the mechanisms behind the apparent exodus of dolphins from the study area at low salinity, it is likely that reduced prey availability is a driver since many estuarine fish species emigrate to higher salinity water during freshwater events (Greenwood et al., 2006; Taylor et al., 2014).
Due to major river inflows and weak tidal influence, most of the Galveston Bay estuary may experience prolonged low salinity throughout the water column with heavy precipitation (Du et al., 2019); however, the HSC acts as a conduit for tidal waters. Water stratification in channels can lead to differences as large as 15 ppt between the surface and bottom (Bechtel and Copeland, 1970). After Hurricane Harvey, the average mid-column salinity in channel habitat was more than 4 ppt higher than in open bay habitat, and habitat-specific encounter rates suggested that dolphins moved toward deeper channels (Fazioli and Mintzer, 2020). However, studies in Pensacola Bay, FL and Barataria Bay, LA found that dolphins exposed to low salinity did not move to areas with higher salinities (McBride-Kebert and Toms, 2021; Takeshita et al., 2021). Future studies should examine fine-scale habitat distribution of dolphins within the Galveston Bay estuary to further evaluate movements in response to flooding.
It is important to reiterate that some dolphins remained in the study area during each freshwater event. Preliminary site fidelity analyses suggest that there is a resident population of dolphins that utilize UGB regularly as a portion of their range (Fazioli et al., 2017). Dolphins that demonstrate high site fidelity within an estuary are known to move within their home range as a response to environmental factors, but are unlikely to abandon it, even in unfavorable conditions (Mazzoil et al., 2008; Wells et al., 2017; McBride-Kebert and Toms, 2021; Takeshita et al., 2021). Dolphins are physiologically adapted to inhabit brackish to oceanic coastal waters with salinities that typically range from 15 to 35 ppt (Ewing et al., 2017; McClain et al., 2020; Booth and Thomas, 2021). Those that remain in an area subject to a low salinity event may suffer from freshwater intoxication due to oral ingestion and/or skin absorption, leading to serious negative health consequences (Ewing et al., 2017; Deming et al., 2020; Duignan et al., 2020; Fazioli and Mintzer, 2020; McClain et al., 2020; McBride-Kebert and Toms, 2021). Effects of freshwater exposure on dolphins can include development of hydropic degeneration and ulcerative or erosive skin lesions (e.g., Wilson et al., 1999; Mullin et al., 2015; Duignan et al., 2020; Fazioli and Mintzer, 2020; McClain et al., 2020; Toms et al., 2020; Townsend, 2020; Takeshita et al., 2021), corneal edema (Deming et al., 2020), and changes in blood chemistry and electrolytes (Ewing et al., 2017; Deming et al., 2020; McClain et al., 2020). Some of these effects were evident when both prevalence and extent of skin lesions increased significantly in the study population after Hurricane Harvey (Fazioli and Mintzer, 2020). Further effects on dolphin health and mortality are likely to occur during freshwater events due to the energetic costs associated with reduced prey availability (Meager and Limpus, 2014; Booth and Thomas, 2021).
Dolphins in UGB are subject to multiple stressors and could be particularly vulnerable to the effects of freshwater (Booth and Thomas, 2021). Epidermal degeneration may heighten exposure to disease and infection, compounded by the increase of pollutants, bacteria and other toxic substances in the water during flood events (Wilson et al., 1999; Kiaghadi and Rifai, 2019; Bacosa et al., 2020; Steichen et al., 2020). Additionally, immunosuppression and adrenal compromise caused by long-term accumulation of toxic pollutants and exposure to petroleum products (Schwacke et al., 2012, 2014) could make dolphins more susceptible to secondary infection and less capable of physiologically adapting to salinity changes in their environment (McClain et al., 2020). More research is needed to understand the population-level effects of freshwater events in Galveston Bay, and to identify which individuals or groups (i.e., age classes and residents) are more susceptible, either physiologically or due to high site fidelity and reluctance to leave the affected area.
In the United States, coastal bottlenose dolphin stocks are protected under the Marine Mammal Protection Act of 1972, and the results of this study have implications for the management of the Galveston Bay stock. Importantly, this study revealed that dolphins use UGB year-round. Continued monitoring is warranted to identify changes in the survival and health of UGB dolphins related to ongoing threats. Seafood advisories, legacy contaminants in sediment, chemical and hydrocarbon spills, and flood events, all make UGB a “high-risk” environment [Houston Advanced Research Center (HARC), 2020; and sources therein]. Heavy precipitation and flood events are expected to increase in intensity due to global climate change (Easterling et al., 2000; Knutson et al., 2010), and as occurred with Hurricane Harvey, these could severely decrease the salinity of Galveston Bay dolphin habitat (Fazioli and Mintzer, 2020). Furthermore, future dredging and infrastructure projects, including the planned widening of the HSC and proposed storm barriers (e.g., USACE and TGLO, 2021), could have considerable short and long-term effects (e.g., noise exposure, increased vessel traffic, and habitat availability). The proposed “Galveston Bay Storm Surge Barrier System” could lead to temporary or permanent changes to salinity and prey assemblages (USACE and TGLO, 2021). The results of our study, emphasizing the year-round presence of dolphins and the importance of salinity, should be considered during the development of these large-scale projects. Mitigation measures will likely be necessary to protect this population, but more information is needed on how dolphins utilize Galveston Bay, the nearshore waters of the Gulf of Mexico, and other Texas bays to identify critical habitats utilized during cooler months and freshwater events.
''These findings should be considered during infrastructure projects (i.e., flood gate system) that may alter dolphin habitat and prey availability.''
''The proposed “Galveston Bay Storm Surge Barrier System” could lead to temporary or permanent changes to salinity and prey assemblages (USACE and TGLO, 2021). The results of our study, emphasizing the year-round presence of dolphins and the importance of salinity, should be considered during the development of these large-scale projects. Mitigation measures will likely be necessary to protect this population, but more information is needed on how dolphins utilize Galveston Bay, the nearshore waters of the Gulf of Mexico, and other Texas bays to identify critical habitats utilized during cooler months and freshwater events.''
