Bottlenose dolphins (Tursiops truncatus) are commonly found in bay, sound, and estuarine environments (Leatherwood and Reeves, 2012; Vollmer and Rosel, 2013; Phillips and Rosel, 2014; Wells and Scott, 2018). These dynamic water bodies, where freshwater mixes with seawater, are among the Earth’s most biologically productive systems (Kennish, 2002). However, due to coastal population growth and rapid development, many estuaries are severely impacted by anthropogenic activities (Kennish, 2002). Bottlenose dolphins that inhabit urban and industrialized estuaries are exposed to diverse stressors including pollution, commercial and recreational fisheries, shipping, dredging and construction, algal bloom, and freshwater inflows (Phillips and Rosel, 2014).

Galveston Bay, Texas is one of the most industrialized estuaries in the United States. Its watershed supports half the population of Texas, and it contains the highest concentration of oil refineries and petrochemical plants in the world [Houston Advanced Research Center (HARC), 2020; GBRC, 2021]. These industries rely on the Houston Ship Channel (HSC) for transportation, leading to heavy year-round ship and barge traffic. Severely impaired water quality prior to 1970 placed portions of upper Galveston Bay (UGB) on the list of the Environmental Protection Agency’s top 10 most polluted water bodies (Youngblood, 2010). Corrective measures have improved water quality over time, but concerns remain over high concentrations of contaminants in sediment and biota [Phillips and Rosel, 2014; Houston Advanced Research Center (HARC), 2020; and sources therein]. Moreover, proposed large infrastructure projects, involving coastal protection barriers (USACE and TGLO, 2021), could further impact wildlife habitat in Galveston Bay.

Prior to this study, most research on common coastal bottlenose dolphins (herein referred to as “dolphins”) in Galveston Bay focused on describing dolphin activity in the southern portion of Galveston Bay, primarily in or adjacent to “Bolivar Roads” (the inlet that connects the Bay to the Gulf of Mexico) (e.g., Henningsen and Würsig, 1991; Bräger et al., 1994; Fertl, 1994; Moreno and Mathews, 2018). This area has been consistently described as having a high concentration of dolphins (Henningsen and Würsig, 1991; Moreno and Mathews, 2018; Ronje et al., 2020). On the other hand, few studies focused on waters of the inner estuary and upper portions of the Bay, and observational surveys conducted in the 1980s–1990s resulted in few or no dolphin sightings in UGB (Jones, 1988; Henningsen and Würsig, 1991; Blaylock and Hoggard, 1994). An apparent “South-North decrease” of dolphins in the Galveston Bay estuary has been attributed to the higher concentrations of prey near the mouth of the estuary, as well as higher levels of contaminants in UGB (Moreno, 2005). Nevertheless, anecdotal data from long-term Galveston Bay users (i.e., boaters, fishers) suggests that dolphin abundance may have increased in UGB over the last few decades (Fazioli et al., 2016). Reconnaissance surveys conducted in 2013–2014 confirmed the frequent presence of dolphins in western UGB and led to the establishment of a long-term monitoring program focused on this previously understudied area.

Increased dolphin activity in this area may reflect the success of efforts to protect Galveston Bay; however, development in the region continues and habitat degradation persists (Youngblood, 2010; Phillips and Rosel, 2014; and sources therein). Accordingly, the Galveston Bay bottlenose dolphin stock [as defined by National Oceanic and Atmospheric Administration Fisheries (NOAA Fisheries)] has been designated as high priority for research and monitoring (Phillips and Rosel, 2014). Information on habitat use, abundance, site fidelity, and stock structure are all needed to better inform management and conservation of these federally protected dolphins (Phillips and Rosel, 2014; Hayes et al., 2019). Moreover, in the face of anthropogenic and climatic changes affecting the Bay, it is imperative to understand the key environmental factors that make the estuary suitable dolphin habitat.

A variety of biotic and abiotic factors influence bottlenose dolphin abundance and distribution. These may include but are not limited to prey and predator distribution, intraspecific social dynamics, topography, turbidity, temperature, and salinity (Irvine et al., 1981; Hastie et al., 2006; Heithaus and Dill, 2006; Mazzoil et al., 2008; Huther, 2010; Hornsby et al., 2017; Moreno and Mathews, 2018). In Galveston Bay, Moreno (2005) observed dolphins further north of Bolivar Roads only June through August, the months with the warmest annual water temperatures. Ronje et al. (2020) indicated a summer increase in overall abundance for Galveston Bay, encountering more dolphin groups in UGB and shallow waters of the estuary during these months, and noting a density shift to deeper channel and Gulf pass habitats during the winter. Anecdotal data gathered from Bay-users also indicated a perceived increase in the number of dolphins present in UGB during summer months (Fazioli et al., 2016). This apparent increase coincides with the months of greater fish diversity in UGB (Bechtel and Copeland, 1970). In terms of salinity, UGB is marginal dolphin habitat, often fluctuating to below estimated 8–11 ppt physiological tolerance thresholds (Ewing et al., 2017; Hornsby et al., 2017; Fazioli and Mintzer, 2020; McClain et al., 2020). A study focused on the effects of Hurricane Harvey, identified a decline in dolphin encounter rates in UGB during the prolonged period of low salinity after the storm (Fazioli and Mintzer, 2020). Herein, we focused on evaluating the relationship between dolphin presence in western UGB and two likely influential abiotic factors: temperature and salinity.

Given the expected continued anthropogenic development in Galveston Bay, we aim to increase knowledge about this understudied dolphin stock to inform future management and conservation efforts. To understand the occurrence patterns of dolphins in UGB, we calculated monthly encounter rates of dolphins (dolphins/km) for four consecutive years. Specifically, we aimed to (1) investigate if dolphins are found year-round in western UGB, and (2) evaluate the effect of temperature and salinity on dolphin presence in UGB.

During the 4 years of this study (January 2016–December 2019), we completed 6655 km of survey effort in 105 field days. This resulted in the observation of 2388 dolphins in 355 groups (Figure 1 and Table 1). The overall average dolphin encounter rate was 0.34 d/km (SE = 0.05). The annual average ranged from 0.28 d/km (SE = 0.06) in 2019 to 0.44 d/km (SE = 0.13) in 2018 (Table 1 and Figure 2). The highest monthly encounter rates occurred in August 2017 (1.09 d/km) and August 2018 (1.23 d/km) and the lowest was 0.00 d/km calculated for 4 months throughout the study (February–April, Table 1 and Figure 2).

The average annual mid-column salinity ranged from 13.74 ppt (SE = 1.38) in 2017 to 9.44 ppt (SE = 1.26) in 2019, with the lowest salinity recorded in June 2016 (1.51 ppt) and the highest in August 2018 (22.37 ppt) (Figure 2). Monthly water temperature ranged from a high of 31.65°C in August 2019 to a low of 9.99°C in January 2018 (Figure 2). The average annual water temperature was lowest in 2018 at 20.79°C (SE = 2.24), while the warmest average water temperature was recorded in 2017 at 23.59°C (SE = 1.56).

The multiple regression equation was highly significant [F(4,43) = 49.02, p < 0.000]. Over 80% of the variance in encounter rates was explained by the predictor variables water temperature and salinity (R² = 0.820). The interaction effect was significant (p < 0.001) confirming the need to include the interaction effect along with the main predictors (Supplementary Material 1). The segmented linear model identified a breaking point in the relationship between encounter rates and temperature, with temperature having a positive linear effect on encounter rates only at over 23.37°C (SE = 1.42, Figure 3). The significant salinity coefficient (p < 0.001) in this model predicted a 0.02 d/km encounter rate increase with every 1.00 ppt increase in salinity (Figure 3 and Supplementary Material 1).

As indicated by the significant interaction effect between the two variables, the simultaneous/joint effect of temperature and salinity on encounter rates was significantly greater than the sum of the parts. The highest dolphin encounter rates in UGB were predicted to occur when both temperature and salinity were at their highest values. This was the case in August 2018 when encounter rates reached the peak of 1.23 d/km (temperature = 29.97°C, salinity = 22.37 ppt; Figure 2). In contrast, encounter rates were below average (<0.34 d/km) in the warm months of June 2016, September 2017, and September 2019 when salinity reached below 3.5 ppt due to heavy precipitation and the corresponding increase in freshwater inflows (Figure 2). This pattern was mirrored in the annual averages when in 2019 the lowest annual salinity (x¯ = 9.44 ppt, SE = 1.26) corresponded to the lowest annual encounter rates (x¯ = 0.28 d/km, SE = 0.06).

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.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

This work was conducted under the National Oceanic and Atmospheric Administration Fisheries (NOAA Fisheries) Scientific Research Permit #18881. Animals were not handled during our field work, so no further reviews were necessary for this non-invasive work.

VM and KF designed and executed the study and wrote the manuscript. KF collected and managed the data. VM performed the analysis. Both authors contributed to the article and approved the submitted version.

The views expressed are those of the authors and do not necessarily reflect the views of supporting organizations.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.