Commentary

Where Have All the Winter Flounder Gone? Digging for Answers

By KENNETH W. ABLE | Feb 14, 2018

Where are the winter flounder going? Is the decline due to rising water temperatures?

As one might expect, “winter” flounder prefer colder temperatures. These fish, which are important in commercial and sport fisheries, come into bays and estuaries such as Great Bay and Barnegat Bay in the fall or winter in anticipation of the cold temperatures found in these shallow waters at this time of year. They also lay their eggs there, on the bottom, at the coldest time of the year, usually around February. Because the temperatures are so cold, the eggs take a long time to develop, often 20 to 40 days, and then the just-hatched larvae continue to develop in the cold temperatures. 

Given that the adults do not like warm temperatures, it is not surprising that they leave and move back into the ocean as the bays and estuaries warm up in the spring, typically around April. We know this from our tagging of winter flounder with acoustic tags that allow us to track them from boats with underwater receivers that can listen for these tags.

Given their preference for cold temperatures, it may not be surprising that as temperatures have warmed in our area over the last several decades, winter flounder are becoming less abundant. The observation comes from several sources. We have measured increasing temperatures at the mouth of our boat basin daily since 1976 and every 15 minutes since 1996, through the efforts of the Jacques Cousteau National Estuarine Research Reserve.

Certainly the fyke netters that fish for the adults in winter in Barnegat Bay know there are fewer around than there used to be. Also, sport fishermen who used to fish off the bridges along Great Bay Boulevard in the Great Bay Wildlife Management Area, and elsewhere, are no longer there in the winter because the fish are not. 

Our own studies at the Rutgers University Marine Field Station, just inside Little Egg Inlet, have seen the same decline. This comes from efforts by graduate students and fresh Ph.D.s led by myself, Tom Grothues and colleagues from other institutions. This includes declines in the abundance of adults in the ocean and the estuaries as well as fewer larvae and juveniles in the estuaries.

All this is to some degree not surprising because winter flounder are near the southern limit of their range in New Jersey. In general, it seems that they are “moving” north as temperatures increase. In reality, it may be that successful reproduction in each bay is becoming less common in the southern part of their range while still viable in the northern part. All this begins to make sense when scientists compare their separate pieces of data from their own areas of expertise. This is one way that science works.

While the decline in abundance corresponds with increasing local water temperatures, the effect may not be direct. Instead it may be that predators, which may be becoming more abundant due to the increasing temperatures, are eating more of the winter flounder than they used to. But it is not the large predators that we typically think of, but very small ones that may be having the most effect.

Our field and laboratory studies have clearly shown that sand shrimp, which reach only a couple of inches in length, are important predators. This makes sense because during the winter and spring when the winter flounder larvae leave the mid-water and settle to the bottom, the shrimp are most abundant. This typically occurs, as we have observed over the last 30 years, in shallow coves on the back bay side of Holgate, for example. As a result, the shrimp predators and the winter flounder prey are occurring at the same time and place, which makes the chance of being eaten much more likely. 

The possible role of predation can be extended because laboratory studies have also demonstrated that small juvenile blue crabs can effectively prey on larger size winter flounder from the spring into the summer. We also wonder whether increasing water temperatures might be shortening the duration of the season for growth of the juveniles. For example, if juveniles grow slowly, they may remain at small sizes longer, and thus are vulnerable to shrimp and crab predators for a longer period of time.

As a result of these studies, we are beginning to gain important insights into how increasing water temperatures, due to climate change, are influencing the number of winter flounder in New Jersey. Unfortunately, the interactions are simply more complex than we would like them to be.  This complexity may be especially so in estuaries where variable temperatures, salinity and dissolved oxygen occur and where tides, day-night variation and human effects are even more typical influences. Of particular importance for this flatfish, and others, is their ability to bury and avoid detection by fishermen and scientists.

All of this continually reminds me of how little we know and how far we have to go to effectively manage winter flounder and other fish populations, especially during a period of warming temperatures.

Kenneth W. Able is a marine biologist and director of the Rutgers University Marine Field Station in Little Egg Harbor. This story is part of an upcoming book by the author on the natural history of the Mullica Valley.

 

 

 

 

 

 

 

 

 

 

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