Fisheries management is complicated as many anglers know. Since scientist have no way of counting all of the fish in the Gulf, or in a given estuary, they have to do stock assessments based on their best estimate of how many fish are removed and how many fish are recruited into the system. This involves not only figuring out how many fish anglers harvest but also how many die naturally (old age, predation, etc.). It also involves knowing at what size a fish spawns, how often a fish spawns, and how many eggs are spawned. And then of course things get really complicated like knowing whether all fish of a same species spawn equally, or how habitat, tides, and currents affects spawning and recruitment. And what about things like red tide events? At the end of the day the more scientist can learn about all of these things the better they are able to accurately describe stocks for management purposes.
To that end the Florida Fish and Wildlife Conservation Commission’s-Fish and Wildlife Research Institute (FWRI) recently published findings on three of our favorite species: snook, redfish, and spotted seatrout. The findings are the result of three separate studies conducted in the Tampa Bay area and designed to capture important information about spawning behavior.
Starting with redfish, of course many anglers know the redfish we target in the estuary are juveniles. Redfish move offshore when they are about three to four years old (around 30 inches) where they join adult schools. Adult redfish spawn in large aggregations near passes in the late summer to early fall. The study FWRI conducted involved tagging both sub-adult fish from Tampa Bay and adult fish from a spawning aggregation. The fish were tagged acoustically and monitored using deployed acoustic arrays established at known aggregation sites near the passes of Tampa Bay and Charlotte Harbor. The study found tagged fish moved back and forth through both acoustic arrays and spawning, as indicated by the length of time in an array was also noted for both areas. An interesting result was that first year spawners (those tagged inside Tampa Bay) arrived to the spawning aggregation much later in the season than the offshore adults. Scientist know that larger female redfish spawn more frequently than smaller adults in what is known as hedge betting. This may partially explain the late arrival of the smaller adults.
Moving onto spotted seatrout, unlike redfish, seatrout is estuary dependent and in particular seagrass dependent for its entire life. Spotted seatrout spawn near passes both in the seagrass and in deeper waters in aggregations. This FWRI study focused on a known spawning aggregation in deep waters near the mouth of Tampa Bay. The study used both acoustic tags and sound production recorders, the later which monitored the seatrout drumming associated with spawning activity. As part of the study researchers determined the sex of each tagged fish and whether it was ready to spawn or had recently spawned. Important findings from this study were that males spawned more frequently (average frequency of 2.2 days) than females (9.3 days), spawning increased near full moons and was at lowest levels between 6 and 7 am. They also determined most fish entered the aggregation from the estuary rather than from the Gulf. Previous tagging studies have shown that seatrout that spawn over seagrass do so because it is in their home and foraging range. In contrast, the spawning aggregation studied here is outside the normal seatrout range. It is thought seatrout go here to increase their reproductive success but do so at a cost (higher chance of predation).
Finally, snook. Snook are part of that less than one percent of all fish that move between fresh and saltwater (diadromous) and in our area are often found way up the Peace River. Along the Gulf coast, snook spawn around passes and along beaches from mid-April to mid-September. It has been reported snook require salinities of at least 24 parts per thousand to spawn in order to activate sperm cells and for the eggs to remain buoyant in the water. The movement from the rivers to the coast to spawn is what’s known as catadromous. This study evaluated the movement and spawning patterns of sexually mature snook over a three year period. Major findings included: Snook spawning increased during full and new moons. Snook exhibit high site fidelity returning to the same spawning site year after year. Some snook skip a year spawning and remain in the rivers. These later results are consistent with those from Charlotte Harbor and the Caloosahatchee. Some snook aggregated in what was likely spawning sites in high salinity waters near river mouths close to optimal habitat and food. It is not yet known whether this results in a shorter spawning season (only when salinity is high enough) or if snook can spawn in lower salinity water than previously thought.
So there you have it, just of few of the good nuggets of information gleaned from these three studies.
Sources:
Susan K. Lowerre-Barbieri, Susan K., Sarah Walters Burnsed, and Joel Bickford. 2015. Assessing reproductive behavior important to fisheries management: a case study with Red Drum, Sciaenops ocellatus, Accepted pre-print. Ecological Society of America.
Lowerre-Barbieri1, Susan, David Villegas-Rı ´os, Sarah Walters, Joel Bickford, Wade Cooper, Robert Muller, and Alexis Trotter. 2014. Spawning Site Selection and Contingent Behavior in Common Snook, Centropomus Undecimalis, PLOS ONE, Vol. 9:7, 16pp.
Lowerre-Barbieri, Susan K., Sarah Walters, Joel Bickford, Wade Cooper, and Robert Muller. 2013. Site fidelity and reproductive timing at a spotted seatrout spawning aggregation site: Individual versus population scale behavior, Marine Ecology Press Series, Vol. 481:181-197.
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