NJ OWL Meeting - Eying Winter Flounder in the Navesink

NJ Outdoor Women's League Meeting
Eyeing Winter Flounder in the Navesink
January 2007

At the first NJ OWL meeting of the 2007 season (to which the public was invited), Dr. Tom Grothues of the Rutgers Institute of Marine and Coastal Sciences gave us a glimpse into the fascinating research he is conducting on the winter flounder in the Navesink River, an estuary that empties into the Atlantic Ocean just south of Sandy Hook. Coauthors of this study are Eleanor Bochenek, Ph.D. and Beth A Phalen, Ph.D. and it is being funded by the New Jersey Department of Transportation under the IBOATNJ program. Similar studies are also being conducted in New Hampshire.

By way of background, Tom gave us some interesting facts about this valuable species.The winter flounder is a member of the order Pleuronectiformes (flat fishes) and the Pleuronectidae family (right eyed). Tom illustrated its appearance with a slide: if you were looking at it lying on its side with its mouth facing in the direction of your right hand, both of its eyes would be on the side facing you.

Dr. Tom Grothues

Although it starts out normal in life, within a month its one eye migrates to the same side as the other; its normal swimming habits change to that of lying on its side on the ocean floor with its both eyes facing up to the surface. Its upper jaw has no teeth and it has a small mouth. Its diet consists of mostly worms, sea cucumbers, and shrimp among other things; in turn, its predators include striped bass, dogfish, blue shark, and cormorants. What is really unusual about the winter flounder, according to Tom, is that it spawns during the winter months, preferably in estuaries whose water temperatures are 0-6Cº. In contrast to most fish whose larvae float in the water column, the winter flounder's eggs, once laid, stick to the bottom. Once hatched they will mature side-by-side with the larvae of the summer flounder.

The spawning habits of the winter flounder come approximately during the same time of the year when marina owners would like to able to dredge the estuaries to keep the waterways open to fishing boats before the waterways get crowded. As Tom explained, the winter flounder are a much valued food source and its continued existence is linked with the success of the fishing commerce in NJ. On the other hand, their habitat is vulnerable to over-fishing and dredging and they are protected under the Essential Fish Habitat designation. EFH is a legal term used to describe a particular habitat (which includes the water column and underlying estuary bed) that is critical for the reproduction and survival of certain fish species. The NJ estuaries fall under this designation and therefore dredging cannot take place during the spawning season.

A winter flounder outfitted with an acoustical transmitter.
Click to enlarge

To ensure a positive outcome, the study was undertaken to answer the following questions: Do the winter flounder use certain areas of the estuary or all of it? What times of the winter months exactly do they use the estuary for spawning? It was decided that using acoustical telemetry (similar to the striped bass surveillance) to track the flounder would be the most logical way to get data to answer these questions. If you remember from the striped bass study, this method involves surgically implanting (under anesthesia) a transmitter whose signals (individual to each fish) are picked up by strategically placed hydrophones and sent to a central location for analysis. In the winter flounder study the tags were attached externally, not implanted, and some tags included sensors, to measure depth and temperature. This year the study includes a GPS system to determine more accurately where the flounder are spawning.

That brought up another question: Would the fish spawn even though they were encumbered by the tags? To answer this question, 2 females and 2 males, that were ripe for spawning, and with tags attached, were placed in a tank of water that simulated the time of year, time of day and the estuary habitat. Tom showed a video of the fish spawning within minutes of being placed in the tank. As he stated, a week later there were thousands of larvae present in the tank. He further explained that a female flounder has the capacity to holding up to a million eggs which she releases at intervals.

The four fish used in the tank experiment were part of a group of flounder that Tom and his associates had caught while going out in an open boat twice a day and during the hours of 10pm to 2am during the winter months. All of the fish were ripe for spawning; (Tom showed a slide of a female that was very swollen with eggs and also explained that the male would exhibit signs if leaking milt.) These fish, 14 in all, were fitted with the acoustical transmitters (attached to the dorsal fin) and sensor tags and returned to the water. The nine hydrophones used for picking up the signals were placed in the shoals of the estuary starting at the mouth of the river. Each hydrophone was numbered; the higher the number, the further up the estuary it was placed. Tom was pleased to report that none of the fish had died and none of the tags were lost although one hydrophone was put out of action possibly by a boater who inadvertently pulled it out.

The resulting data gleaned from the 61,415 contacts from 8 of the hydrophone showed that more contacts occurred between sundown and before sunrise, revealing the fact that the fish were moving, i.e. spawning during the nighttime hours. It further showed that the fish moved up the river in early to mid-February and also early to mid-March. Most had come back to the entrance of the estuary by April; signaling that spawning had come to an end. It was significant that the hydrophones placed further up the estuary picked up fewer signals and that #9 which had been placed the furthest upriver where the water temperature was warmer picked up none.

Tom explained that the number of fish that were caught was quite low probably owing to the warmer water temperatures in the last few years. However, fewer fish in a particular environment will seek out the best habitat and there will be little variation of their movements. For the second year of the study it is planned to place the hydrophones near Red Bank, which has deepest sections as well as the most shallow parts of the estuary.

Tom emphasized the need for the public to be aware of this ongoing program and in light of that encourages anyone interested to go online to http://marine.rutgers.edu/navesink to get a general overview of the project and view the photos.

We thank Tom again for his willingness to give of his time and knowledge to enlighten us in yet another fascinating way of learning about wildlife.

-- Sandy Norman