A new study in Marine Biology shows how clearly young bull sharks change their use of the Galveston Bay system with the seasons. In summer and autumn, the tagged animals were frequently in the large river estuary. In winter, they were hardly detected there at all; individual sharks migrated south as far as the Aransas Bay. The longest recorded distance was 259 kilometers.
The study area is located in the US state Texas on the northwestern Gulf of Mexico. Estuaries such as Galveston Bay connect rivers, bays, and coastal waters. For young bull sharks, such transitional areas are important habitats, and at the same time, temperature, salinity, and food availability change greatly there. The study makes it clear that for protection, not only individual bays but also the connections between them are relevant.
Three types of robbers tracked acoustically for two years
The research team equipped 20 bull sharks, alligator gars (Atractosteus spatula) and Atlantic stingrays (Hypanus sabinus) each with acoustic transmitters. Receivers in the Galveston Bay Complex and in neighboring waters recorded the individual signals between September 2023 and September 2025. Within the bay, 13 alligator gars, twelve bull sharks, and eleven stingrays were detected at least once.
A total of 16,048 valid detections were included in the evaluation. Bull sharks accounted for 75 percent of these detections and were on average recorded at more receiver stations than the other two species. All recorded bull sharks were juveniles. At the time of tagging, they measured between 92.6 and 152 centimeters, with an average of 122.2 centimeters.
However, many detections do not automatically mean many sharks. One animal can be recorded repeatedly at the same station, while another leaves the range of the receiver network. Therefore, the study compares movement and space-use patterns; it is not a population count.
In winter, bull sharks moved south
Of the bull shark detections, 43.1 percent occurred in autumn and 36.8 percent in summer. In spring it was 19.7 percent, in winter less than one percent. Seven of the 20 tagged bull sharks were recorded outside of Galveston Bay Complex. All records were south of the bay, including in Matagorda Bay and Aransas Bay.
Four of these seven animals, or 57 percent, later returned to the Galveston Bay system. A 103.5-centimeter-long bull shark left the West Bay in October 2023, was identified in December in Matagorda Bay, and reappeared in March 2024 in Galveston Bay. This single course illustrates the seasonal connection, without proving that every juvenile takes the same route.
The authors view the winter absence in line with previous work on the influence of water temperature on bull sharks. However, temperature, salinity, prey, and reproduction were not tested as causes in this study. They remain plausible explanations for the observed seasonal shifts, not directly proven triggers.
Bull sharks and alligator gars shared particularly a lot of space
Using a network analysis, the researchers examined which animals were recorded at the same station within a 24-hour time window. The strongest overlap between species was observed between bull sharks and alligator gars. With Atlantic stingrays, the spatial overlap was lower.
Even within their own species, bull sharks overlapped particularly strongly. The researchers suspect that this could increase competition and favor a size-dependent division of habitat. This is an ecological interpretation of the location patterns; direct competition or displacement was not observed.
Alligator gars remained, stingrays were rarely observed
Alligator gars were detectable in Galveston Bay Complex throughout the entire year. Two animals migrated into the Brazos estuary, one of them about 18 kilometers upstream. After this relocation, both were no longer recorded in Galveston Bay during the study period.
The Atlantic stingrays were detected almost exclusively in summer and only in West Bay. No reliable year-round area affiliation can be derived from this: 88.2 percent of their detections occurred in the first two months after tagging. Range and location of the receivers, possible transmitter loss, and predation may have additionally influenced the picture.
Why West Bay should be interpreted with caution
For all three species, the majority of the locations were in West Bay. Many animals had been tagged there, and part of the receiver network was also concentrated there. The authors therefore warn against extrapolating the heavy use of West Bay to the entire populations without reservation.
Acoustic telemetry also does not provide a continuous GPS track. It shows when an animal came within range of a station. Paths between two receivers remain unknown, and a missing signal can indicate emigration, a network gap, transmitter loss, or the death of the animal. The small number of detected individuals also limits generalization.
Protection needs connections between bay and coast
Despite these limitations, the pattern is revealing for management: young bull sharks do not rely on a single nursery, but instead move seasonally between estuaries and coastal bays. Protective measures, fishing regulations, and the assessment of construction or coastal projects should therefore take into account migration corridors and neighboring bays.
The study thus underscores a principle of ecosystem-based management: It is not only important where a species is frequently recorded, but also which routes it requires throughout the year and with which other predators it shares habitats. For Galveston Bay, longer, spatially broader telemetry series are the next step to examine the now visible patterns.
