Spotted Seatrout Gill Net Survey

Scientists  and local anglers have documented a decrease in the spotted seatrout population in lower Tampa Bay following the red tide event in 2005 (Article: 2005 Red Tide Impacts on Fish Spawning in Tampa Bay) .

Although it is known that spotted seatrout were killed and/or displaced by the red tide, the time it takes for the population to recover is not known. By comparing catches and the biological condition of spotted seatrout before and after the red tide event, biologists can document the recovery process.

The gill net survey is a small component of a comprehensive research plan examining spotted seatrout in Tampa Bay (Article: Spotted Seatrout Research in Tampa Bay-An Overview). The gill net survey originally began in 2000 and extended through 2003, covering a variety of areas in lower Tampa Bay.

Adult spotted seatrout were targeted for capture and sacrificed to collect data on age, reproductive condition, and health.

Captured spotted seatrout are immediately placed on ice for
further study. Other species are released alive and unharmed.

Based on these three years of data (2000-2003) scientists gained a much better understanding of the population in Tampa Bay (demographics and reproductive biology) during normal conditions. After the major red tide event in 2005, the gill net survey was reinstated to collect fish and compare data between pre- and post-red tide periods. Although the original gill net survey encompassed a much broader geographic region, the post-red tide survey focused on a smaller area that has been heavily monitored through a variety of studies. From these studies and from the original gill net survey, biologists knew this area traditionally supported a healthy spotted seatrout population.

Following the 2005 red tide, catches by both biologists and recreational anglers in this area dramatically declined. It was apparent that the red tide had either killed or displaced the once healthy spotted seatrout population but the extent of the damage, as well as how quickly the fish would rebound, remained unknown. It is necessary to capture fish after the red tide event to evaluate if the recovering population is made up of fish that survived the red tide, or fish that were born after the red tide event.

This can be accomplished if the ages of the fish in the recovering population are known.  In order to determine age, spotted seatrout must be sacrificed, as age is determined by counting the rings within their otoliths, or earstones,  (Articles: Introduction to Aging Fish: What Are Otoliths? and The Otolith Sectioning Process). However, in an effort to kill as few fish as possible and to learn everything possible from the fish that are sacrificed, multiple types of data are collected from each fish.  This data contributes not only to our understanding of the spotted seatrout recovery but to a number of different studies being conducted at the Fish and Wildlife Research Institute such as: trophic interactions (gut contents), fish health (gill arch and internal organs), and genetic stock structure (fin clip).

Spotted seatrout are measured, weighed, and the circled organs are used to determine various conditions. Further detail on the fish health analyses can be found in the article: Fish and Wildlife Health Program Summary.

Although the catches of spotted seatrout in 2006 and 2007 are still below pre-red tide levels, the numbers are increasing, indicating the population is on the road to recovery. Spotted seatrout mature at a relatively small size and age, with most fish spawning at age one, and approximately 11.8-15.7 inches long (Article: Spotted Seatrout (Cynoscion nebulosus) Species Account). Because spotted seatrout reproduce at an early age and produce hundreds of thousands of eggs, they are expected to be fairly resilient and capable of recovering in the near future.