Osmoregulation in Estuarine Fishes

It might come as no surprise to some people that Texas’ coastal bays are constantly changing. Climate, seasons, weather, tides, and anthropogenic factors are all continuously altering the physical, chemical, and biological aspects of these waters. One of the many elements that affect a marine ecosystem is salinity. Texas Parks and Wildlife’s Coastal Fisheries division monitors environmental factors - including salinity - in its routine sampling of all bay systems. Freshwater input from rivers, precipitation, and tidal interactions create an environment that requires local fish, shellfish, and even birds to find some way of surviving fluctuating salinities. Fish living in our bays, including some of Texas’s most popular sportfish species, use a biological process known as osmoregulation to make the most of their estuarine habitat. Osmoregulation allows these fish, along with a myriad of other marine organisms, to survive (and even thrive) in such dynamic waters.

Salinity levels vary from mostly marine to almost freshwater conditions depending on seasonal and locational changes to freshwater/saltwater inputs. Freshwater is considered less than 1 parts per thousand (ppt), while ocean water is usually considered 35 ppt, with other states of water including brackish water that is a mixture of salt and fresh and hypersaline water that is saltier than regular ocean water.  In Texas bays, there is generally a salinity gradient from north to south, with Sabine Lake on average being the freshest marine ecosystem and the Lower Laguna Madre being the saltiest. This salinity gradient is due primarily to higher annual rainfall patterns observed in the northern bays, and higher evaporation rates combined with less freshwater input in the southern bays. With the spring and summer months underway, the Texas coast will see more precipitation than in other times of the year. Spring rain and summer weather events, such as hurricanes, both contribute to immediate input of freshwater from rain and higher inflow from rivers and runoff, thereby decreasing salinity. Conversely, lower precipitation, incoming tides, and evaporation contribute to increases in salinity. Salinities observed by TPWD from 2000 to 2021 show that all bays exhibit near-freshwater salinity levels at some point, while select systems, such as the Upper Laguna Madre (including Baffin Bay) show instances of extreme salinity around 90 parts per thousand (ppt; Figure 1).

So what does this mean for the fish having to contest with such wide salinity ranges? Well, it all comes down to how estuarine fish osmoregulate. Osmoregulation is the process through which an animal maintains preferred fluid (water) and ion (salt) concentrations inside the body, regardless of its surrounding environment. Two contrasting examples are seen in fishes that live strictly in either fresh or marine waters. These organisms are known as stenohaline, meaning they’re adapted to a narrow salinity range. In freshwater species, salt concentrations are higher inside the body than in the surrounding water. As a result, freshwater fish must rid themselves of excess water while retaining salt. They do this by excreting a large amount of dilute urine to retain as much salt as possible. In marine fish, the opposite occurs. Marine fish have a lower salt concentration inside the body than the surrounding seawater and must rid themselves of excess salt while retaining water. They do this by drinking sea water and excreting a small amount of concentrated urine. Osmoregulation requires energy from the fish, so it may not always be the best available option for navigating changes to salinity. To avoid osmoregulation, some species will steer clear of wide salinity fluctuations altogether by moving with the changes in the salinity gradient. Many species of catfish like blue and channel, though tolerant of some salt, tend to be found where the salinity levels are lower and where rivers flow into bays as their primary habitat is freshwater. When bays experience a large inflow of freshwater, catfish can often be found over a greater range as the area of lower salinity has increased. However other species are adapted to tolerate wider salinity ranges (known as euryhaline) and can make the most of their constantly changing environment.

Three well-known examples of euryhaline fish are Spotted Seatrout, Red Drum, and Black Drum. TPWD gillnet catch rate data for these three species can be graphed showing their presence from 0-70 ppt salinity (Figure 2). In the graph, Red Drum and Spotted Seatrout show a relatively consistent catch rate across the salinity gradient. Red drum specifically show a minor negative correlation, seemingly preferring fresh to normal ocean salinities before declining in abundance in hypersaline environments. Spotted seatrout gain abundance towards the 20-25ppt salinities, decline slightly towards standard ocean salinity, but show a peak towards hypersaline environments. Black drum increase in abundance peaking significantly in hypersaline environments with a minor decline from around 50 ppt to 70 ppt salinity. Black Drum are especially known for their spring spawning run where they seek conditions from 18ppt to full-blown saltwater, their eggs preferring polyhaline (18-30ppt) conditions for survival. In most cases, Red Drum and Spotted Seatrout will seek similar conditions for spawning. However, TPWD has successfully stocked Red Drum in several freshwater cooling reservoirs, low salinity impoundments, and inland systems, exhibiting the capability for even early life stage fish to be adaptable to an incredible range of conditions. All the while, this is the same species of fish that can be caught off the jetties of our coastline or gulf shores. Trout can be found from hypersaline all the way into the fresher upper bays in later life stages as they search for food sources that change with seasonal availability.

So why does this matter to anglers? All three species discussed above have preferred water chemistry conditions for spawning, larval survivability, food availability, and allocated energy expenditure to exist in any one salt concentration for a given amount of time. Therefore, aggregations or assumed locations of these target species can be partially attributed to the fish’s needs and how salinity plays into that. Many anglers like to capitalize on the opportunity to target black drum during the spawning run when larger individuals are found together in the lower parts of our bays towards the gulf. In other times of year, Black Drum can just as easily be caught by anglers on rod and reel or trotlines in upper bay areas where river inflow makes the waters far less saline. These preferences, depending on time of year, life history, and need, mean that anglers can track down their favorite fish across the dynamic salinity gradients of our bay systems – and we have osmoregulation to thank for it. This adaptability is yet another reason to celebrate these three iconic sportfish that call our Texas bays home.