Salinity and Water Temperature as Predictors of Bottlenose Dolphin (Tursiops truncatus) Encounter Rates in Upper Galveston Bay, Texas

Abstract

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.

Site fidelity of common bottlenose dolphins (Tursiops truncatus) in a highly industrialized area of the Galveston Bay estuary

“…Many of these individuals are likely using areas in lower Galveston Bay during the cooler months and moving north seasonally. A similar pattern has been observed in other estuaries, with dolphins using the inner estuary more frequently in warmer months and shifting to pass and coastal areas in winter (Irvine et al, 1981;Scott et al, 1990;Vollmer & Rosel, 2013); prey migration has been proposed as the driver for these seasonal movement patterns (Irvine et al, 1981;Mintzer & Fazioli, 2021;Scott et al, 1990;Vollmer & Rosel, 2013;Wilson et al, 1997).…”

“…Dolphin groups were encountered in all three habitat types surveyed: near-shore, open bay, and deep channel (Figure 1). The number of new distinct identifications peaked during the warm months (Figure S9), a pattern that corresponded with the significantly higher encounter rates of dolphins observed in the study area during these months (Mintzer & Fazioli, 2021). Of the dolphins observed during the monthly standardized surveys in our primary study area (Figure 1), 442 had adequate natural markings to allow for individual identification and were included in the site fidelity cluster analysis.…”

“…The primary study area was a 198 km 2 area in the northwest segment of UGB (Figure 1). Standardized monthly boat-based photo-ID surveys were conducted in the primary study area between January 2016 and December 2019 (Figure 1, Table 1, Figures S1-S8; Mintzer & Fazioli, 2021). Due to the large study area, surveys typically spanned two days and were completed on consecutive days, weather permitting.…”

“…A resident population of about 200 individuals was identified near Bolivar Roads and the Galveston Ship Channel (Bräger, 1992(Bräger, , 1993Fertl, 1994). Since the 1990s, perhaps due to habitat modification and/or improvements in water quality, dolphins seemingly began to inhabit a greater portion of the estuary, and they are now frequently found in UGB (Mintzer & Fazioli, 2021). Abundance estimates for the entire stock fluctuate seasonally and are estimated to be 850 in the winter and 1,250 in the summer (Ronje et al, 2020).…”

“…In 2013, the Galveston Bay Dolphin Research Program (GDRP) began to conduct dolphin photo-identification (photo-ID) boat-based surveys in western UGB, and subsequent monitoring determined that dolphins can now be found in UGB year-round (Mintzer & Fazioli, 2021). As a next step, we sought to investigate if the same individuals regularly use UGB, and if there are individuals with varying site fidelity to the upper portions of the estuary.…”