Spotted Seatrout Research in Tampa Bay—An Overview
The spotted seatrout (Cynoscion nebulosus) is an estuarine-dependent species that supports one of the largest and most popular inshore sport fisheries along the southeastern United States and in Florida. Because of this, the species is highly regulated in Florida, with minimum and maximum size limits, a daily recreational bag limit, and a closed season.
(Visit Saltwater Fishing Regulations for the history of spotted seatrout regulations, as well as a more detailed description of the current regulations).
It is important that we understand where, when, and what affects the quantity that spotted seatrout spawn. The current trend in Florida coastal population growth will impact fish that spawn close to shore, both through increased fishing pressure and spawning habitat degradation. Tampa Bay is Florida's largest and most industrialized estuary, and because spotted seatrout spawn primarily in estuaries, this is an ideal location to begin mapping spotted seatrout spawning habitat. Because Florida spotted seatrout are managed based on spawning potential ratios (Spotted Seatrout Stock Assessment), there is also a need for a better understanding of spotted seatrout reproductive biology. Specifically, we need to understand how many eggs a female spotted seatrout will produce in a spawning season and what factors may affect the number of eggs produced. This type of information is critical if we are to protect spawning fish, allowing them to produce future generations.
The Florida Fish and Wildlife Conservation Commission's (FWC) Fish and Wildlife Research Institute (FWRI) is currently working on a number of research projects to address these questions. Specifically, they address three issues: spotted seatrout fecundity (the number of eggs a female produces) and what may cause it to vary (such as age or size of the fish, environmental variables, and differing spawning sites); mapping spawning sites throughout Tampa Bay and the nearby passes in an effort to understand essential spawning habitat ( Article: Mapping Spawning Habitat of Spotted Seatrout (Cynoscion nebulosus) in Tampa Bay), and; conducting detailed research at a known spawning site to determine how individuals behave or use a particular site and what factors may affect their behavior (such as red tide).
Spotted seatrout are multiple spawners, meaning they spawn many times throughout an extended spawning season that usually begins in mid-March and runs through mid-September in Tampa Bay. They are believed to spawn primarily in the estuary that they were born in. In each spawning event or batch, they release hundreds of thousands of untended, pelagic eggs which are approximately one millimeter (mm) in diameter.
Because spotted seatrout spawn at dusk, weekly collections in lower Tampa Bay were made in the morning (to collect fish which had spawned the night before) and in the evenings (to collect fish that would spawn that night) using both hook and line and gillnets.
Fish were kept on ice and transported back to the FWRI lab, where they were measured and weighed. The fish's ear stones or otoliths were extracted. Otoliths are boney structures which can be sectioned and used to age the fish (rings are laid down each year, similar to a tree; Article: Introduction to Aging Fish: What Are Otoliths?). The gonads, or reproductive organs, were also removed, weighed, and assessed as to their level of development. Some females from evening samples contained gravid ovaries, an ovary which contains hydrated oocytes (eggs prior to ovulation) just prior to their release. Because no eggs have been released, these ovaries can be used to estimate how many eggs a fish would spawn in that batch, referred to as batch fecundity.
The contents of gravid ovaries are washed under high pressure to separate the oocytes and remove them from the ovarian wall. These oocytes are then preserved in formalin, so that batch fecundities can later be estimated. To estimate batch fecundity, a scientist takes a small, weighed sample of the ovarian tissue and counts the number of hydrated oocytes under a microscope. They do this for two samples, and then determine the average number of eggs per gram of ovary. Based on the weight of the ovary, they can estimate the number of eggs a fish that size would spawn.
To understand how many eggs an individual spawns over the spawning season (called annual fecundity), it is necessary to determine both batch fecundity (the number of eggs released on any given night) and spawning frequency (the number of times a fish spawns within the spawning season). Spawning frequency estimates are based on the percentage of actively spawning females collected throughout the spawning season. There are two means of identifying active spawners. One is based on gravid females that would spawn that evening (called the percent hydrated method); the other is based on females that spawned the night before (called the postovulatory or POF method). In this study only the POF method was used to determine spawning frequency. The percent hydrated method was not used, as spotted seatrout are known to aggregate to spawn. This behavior leads to a larger percentage of active spawners occurring at a spawning site than is representative of the population and thus this method might give biased results.
Spotted seatrout collected from the Tampa Bay area ranged in size from 145 to 640 mm (5.7-25.2 inches) total length, with an average size of 354 mm (13.9 inches) total length. Ages ranged from age less than 12 months old to eight years. Most fish (93%) were younger than age five. Most female spotted seatrout were mature and capable of spawning by age one and all by age two (at approximately 350 mm total length or 13.8 inches). Spawning seatrout ages one through eight were observed. Spawners age two to six were more common in the actively spawning population than in the general population, indicating that older fish may spawn more often than younger seatrout.
The number of eggs spawned per evening (batch fecundity) increased with female body size and age, and could be up to a million eggs per female. Based on the percentage of females collected in the morning with POFs, spotted seatrout were estimated to spawn once every nine days, or approximately 21 times over the season. Annual fecundity increased with age, ranging from roughly two million eggs for females that are one-year old to 18 million eggs for females ages five and older.
This initial study on spotted seatrout fecundity produced estimates of important reproductive parameters, but results from this study also suggested that spotted seatrout fecundity could vary over small spatial scales, and that spotted seatrout spawning habitat was highly variable.
Because of these results, it was clear that to understand spotted seatrout spawning habitat it would be necessary to sample a larger area. With this in mind, we initiated a study (2003-2005) to determine spotted seatrout spawning habitat throughout the Tampa Bay area. Because male spotted seatrout make courtship calls associated with spawning, we were able to use these sounds to determine spawning activity.
We also initiated research to look in detail at the reproductive dynamics of a spawning site. To evaluate the level of spawning activity at this site, we monitor spotted seatrout courtship calls using a long-term acoustic recording system throughout the spawning season. We also monitor the presence of sonically tagged individuals to determine behavior and site fidelity.
Each implanted fish can produce important data about spotted seatrout behavior at the spawning site. However, they can only produce this information if they survive the spawning season. To aid in this research, we ask that anglers release any spotted seatrout that have an external dart tag near the dorsal fin. This tag indicates that this is a fish implanted with a sonic tag. It is also possible that some fish will shed their dart tags, and an angler might not realize they have kept a tagged fish until they notice a small black cylinder while cleaning the seatrout. If you capture a tagged fish please contact the tag return hotline at 1-800-367-4461 or email tagreturn@myfwc.com and let us know the tag number, date, time, and location where the fish was caught. Also, please include the bait that was used, and whether the fish was kept or released. And please release spawning (roed up) fish so that they can produce future generations of fish for everyone to enjoy.