Rhode Island Saltwater Fishing
Spatial Sex-Segregation in Rhode Island Fluke
Graduate School of Oceanography
For many Rhode Islanders, the opening of fluke season on May 1st is a sure sign of summer. Fluke, or summer flounder, support one of the most important commercial finfish fisheries on the Atlantic coast and one of the largest recreational fisheries in the United States (NMFS 2018). In fact, the recreational fishery is so significant that it is allocated a significant portion of the total annual fluke harvest, on par with the commercial fishery (NEFSC 2013). While fluke are managed as one coastwide stock, recreational harvest limits vary among states or groups of states (Terceiro 2018). This framework was created to allow states flexibility in how they meet their harvest limits for their respective recreational fisheries. However, it is important to consider fluke biology in developing these rules each year.
Like many flatfish, fluke are sexually dimorphic. This means that the sexes are visibly different from each other. Specifically, females grow larger and faster than males (King et al. 2001). When recreational harvest is regulated by a minimum length limit, as is the case in Rhode Island, this dimorphism creates a risk of removing a disproportionate number of the females that are important for stock productivity. A series of studies in New Jersey conducted between 2009 and 2016 illustrated this possibility. The researchers showed that the vast majority of fluke harvested by New Jersey anglers were female and as a result, went on to suggest a slot limit as a viable management alternative for the recreational fishery (Morson et al. 2012, 2015, 2017). However, these investigations also showed something fishy was going on- the sex ratio of a boat’s catch varied depending on where it came from. Fluke landed in shallow waters seemed to be female more often than those caught in deeper habitats. While it has been observed in other flatfish like Pacific halibut (Loher et al. 2012) and American plaice (Swain 1997), spatial segregation of the sexes was not known previously in fluke. Furthermore, it was difficult to pull apart potential patterns of sex-segregation from patterns of fishing effort and angler behavior.
In order to get to the bottom of this phenomenon, a study was launched by researchers from the University of Rhode Island Graduate School of Oceanography (GSO) and the Rhode Island Division of Marine Fisheries (RIDMF). Rhode Island is unique in that the state is swimming in scientific survey data of its marine ecosystems. The weekly trawl survey conducted by GSO since 1959 is the longest of its kind in the western hemisphere. In addition, RIDMF has conducted monthly and seasonal fish trawls at stations throughout Narragansett Bay and the Rhode Island and Block Island Sounds since 1979. Utilizing these two surveys, over 1,300 fluke were collected throughout Rhode Island state waters between May and October of 2016 and 2017. Each fish was measured and dissected to determine its sex. The proportions of each sex in each trawl were then compared to a suite of potential parameters, like bottom water temperature, month, and depth, to look for evidence spatial sex-segregation and understand what factors may influence it.
The results of this study showed that fluke harvested by recreational anglers in Rhode Island are indeed almost entirely female. For example, under the 18 in and 19 in minimum length limits used in the Rhode Island recreational fishery in 2016 and 2017 when the study was conducted, 93.0% and 97.7%, respectively, of the sampled “legal-sized” fluke were female. The size distribution of fluke in state waters was also found to vary throughout the season. Smaller fish were the first to arrive in May before large females reached the coastal zone in late-June and July. The large fluke then began to thin out in August as they presumably headed offshore to spawn. Interestingly, young-of-the-year fluke were also observed in the trawl samples. After being spawned in the fall and spending winter and spring growing in the shallow areas of Narragansett Bay and the coastal ponds, young-of-the-year fish appeared to move to deeper waters beginning in July. By October, these young fluke made up a large proportion of the fish remaining in state waters before they too migrated offshore.
Clear patterns of spatial sex-segregation were observed in the sampled fluke. Females were found to prefer shallow waters while males dominated deeper areas of the coastal zone. It is not known what causes these patterns, but it may be because shallow habitats are warmer and thus help females to maintain their fast growth rates. That said, the sex ratio was not observed to respond to every change in water temperature. Further research will be needed to better understand why female fluke preferentially select shallower habitats.
The degree of sex-segregation also changed throughout the season. The catch in May tended to be dominated by small female fluke, before more males and large females moved inshore in June. These males and large females then moved offshore together beginning in August, leaving a population heavily skewed toward young female fluke by October (Figure 1). Samples from locations less than 50 ft deep were female-dominated throughout the season, while locations deeper than 50 ft were male-dominated in every month except October.
Thinking from the angler’s perspective, these patterns combine to suggest a clear fishing strategy to find legal fluke. The proportion of fluke in the trawl samples legal for recreational harvest peaked in July (Figure 2). At locations less than 50 ft deep, nearly 40% of the July-captured fluke were larger than the 18” minimum length limit used to regulate the recreational fishery in 2016! All of the sampled “doormat” fluke (here considered fish >24 in) were also observed between mid-June and mid-August. If, however, you find yourself trying to catch those last few legal fluke late in the season, you will want to head for deeper waters. Deep areas of Rhode Island state waters become warmer than shallow habitats in October as Fall cooling begins to take effect.
In addition to identifying and characterizing patterns of spatial sex-segregation in fluke, a statistical model was constructed to predict the probability that captured summer flounder were female based upon their individual total lengths, the depth of the capture location, and the month of capture. The model was found to predict sex correctly in individual flounder nearly 80% of the time. When the model was applied to a large sample of fluke, at the scale of annual recreational catch for example, assigning each fluke proportionately between the sexes based upon the predicted female probability produced a very accurate estimate of the sample-wide sex ratio. In this manner, the model could be used to accurately predict the sex ratio of fluke harvested within Rhode Island waters using capture information that is commonly available to fisheries scientists. However, it is unclear how well the model would perform outside the immediate area. More research needs to be conducted in other locations before the results found in Rhode Island are used in fluke management coastwide. That being said, the clear and predictable patterns of fluke sex-segregation identified in this study suggest that implementation of more targeted spatial fluke management measures to preserve the female spawning stock may be possible in the future.
If you would like to learn more about this research, it was published in February 2019 as an open access scientific paper in Marine and Coastal Fisheries under the title “Evaluating Summer Flounder Spatial Sex-Segregation in a Southern New England Estuary” (https://afspubs.onlinelibrary.wiley.com/doi/10.1002/mcf2.10065). This work was a contribution of the Rhode Island Marine Fisheries Institute and benefitted from monetary support of one of its participants by the National Science Foundation REU Program (OCE-1460819) hosted by the GSO Summer Undergraduate Research Fellowship in Oceanography (SURFO).