research news |
February 14, 2008- X-Tag recovered on Cape Hatteras by Alexis Hodges! Full story found here.
August 22, 2007- New paper published in Fisheries Bulletin by Large Pelagics Research Lab PhD candidate Walter Golet and collaborators. Paper can be found here. Also, press articles can be found on our industry news page.
Atlantic bluefin tuna (Thunnus thynnus) with their massive size and tremendous power, have fascinated fishermen and non-fishermen for decades. A voracious predator (often referred to as pigs with fins), bluefin tuna in the western Atlantic inhabit the waters of the Gulf of Maine and Canadian Maritimes where they feed on small pelagic fish such as herring and mackerel. During their five to six month (late May to November) sojourn on these feeding grounds bluefin tuna accumulate substantial amounts of fat which they utilize for migration and reproduction in the following months.
The physiological and morphological adaptations of bluefin tuna that make them an efficient predator (capable of adding 5-7% or more of their body weight a month), are the same characteristics which make them appealing to fishermen. During the 1930's and 1940's most tuna fishing was done for sport. Large-scale commercial fishing started in the late 1950's where smaller fish were targeted by purse seiners for canning and animal feed. Bluefin tuna value during these early years was minimal, with most fish being sold for a few cents per pound. However, during the late 1970's and early 1980's the Japanese sushi and sashimi markets started to seek out larger and fatter bluefin tuna. Bluefin captured in New England were particularly appealing to buyers due to the large fat reserves (which made the meat taste better, analogous to the marbling in beef) typically acquired in this region by September. Fueled by an unquenchable appetite and booming economy, Japanese buyers began purchasing bluefin at incredible prices, leading to a mini commercial fishing gold rush in New England. It was not uncommon to average $12.00-$15.00 or more per pound for most fish brought to market.
Unfortunately, this boom was short lived. During the last 10 years many different industry representatives including the fishermen themselves and buyers started to observe that bluefin tuna were no longer accumulating the highly desired fat reserves they had in previous years. This resulted in lower average prices ($2.00-$7.00 per pound) and in some cases, fish would not even sell at market.
We decided to investigate these claims of declining quality by examining commercial log books containing the grade data for approximately 4000 bluefin caught from 1991-2004. These logbooks contained the quality records for each fish brought to the dock. Grades (C, C+, B-…A) are assigned to four characteristics freshness, meat color, fat content and shape. These grades act as a guide to the dealer for the fish's potential market value. However, they can also be used to evaluate somatic (body) condition. Our analysis indicates significant changes have occurred in the fat content and shape of bluefin tuna in the Gulf of Maine over the last 14 years. They key finding being, fish caught in August, September and October no longer contain the fat reserves they did in the early 1990's.
The model we used looks at the probability of catching a fish of a particular grade during a given month for a specific year. Our results showed the probability of catching a good quality (high fat content) bluefin tuna has declined significantly, while the probability of catching a lower quality bluefin has increased (Fig 1). In figure 1, poor quality fish (C and C+ ) now comprise approximately 80% of the catch (2004) while good quality fish (B or better) are less than 1% of the total. Also, fish appear to be arriving in the Gulf of Maine in a more energy depleted state than in the early 90's (pink line for the C grade). Similar results were obtained when we analyzed the shape data (Fig 2).
Aside from its importance to commercial value, lipids are especially important for bluefin as they provide the energy to sustain long distance migrations and support their large reproductive output. If bluefin tuna can no longer accumulate these lipids it may indicate that changes are taking place in the Gulf of Maine ecosystem which could be far more reaching than just bluefin tuna quality, such as changes to migratory and reproductive patterns.
Currently, we are working on trying to identify what factors may be responsible for this decline in quality. Our current analyses are directed at examining two more extensive bluefin databases in conjunction with available prey data.
July 1, 2007- New paper published in Marine Biology by Large Pelagics Research Lab PhD candidate Jennifer Goldstein and collaborators
J. Goldstein, S. Heppell, Andrew Cooper, Solange Brault, Molly Lutcavage. 2007. Reproductive status and body condition of Atlantic bluefin tuna in the Gulf of Maine, 2000-2002. Marine Biology, 151(6): 2063-2075
This study is the first to use current histological techniques to determine the reproductive condition of the Gulf of Maine bluefin assemblage, and the first to attempt to link condition indices to reproduction for the western Atlantic stock. The long-standing paradigm is that all Gulf of Maine bluefin tuna are members of a western stock that matures at 8-10 years of age and spawns in the Gulf of Mexico, but recent electronic tagging results challenge that assumption. This study gives direct biological evidence that bluefin may be on a more variable or asynchronous reproductive schedule, possibly spawning in areas outside of known spawning grounds, or skipping a year entirely. Given that the spawning stock biomass is estimated to be at 13% of MSY, these results have important implications for population assessments and management of this species. Several hypotheses are advanced to explain the variability in the reproductive status of bluefin entering the Gulf of Maine, including (1) variation in the rate at which fish of different sizes/ages resorb atretic follicles, (2) arrival from different areas of the Atlantic, and (3) skipping a reproductive event.
The range of post-reproductive states present well outside of known spawning areas from this study and from historical cruise data combined with recent electronic tagging results lends evidence to a more asynchronous model for bluefin tuna reproduction than previously assumed. Males of commercial sizes (185–263 cm CFL) and small females (185–235 cm CFL) arriving in June and July in mature or mature-inactive condition may indicate that spawning has occurred outside of known spawning areas, or spawning has been omitted during the last maturity cycle (Fig 1). This result is particularly striking given that the mean age of electronically tagged bluefin tuna present on the spawning grounds are ages 11 and above (=241 cm CFL) (Block et al. 2005 ) and recent analyses on longline data in the Gulf of Mexico estimate the age of 50% maturity to be 12 years (Diaz and Turner 2006), although this finding may be highly biased by size selectivity of longline sampling (Davis and Farley2001; Corriero et al.2003). The existence of oceanic spawning areas remains a possibility, and must be addressed by more extensive sampling, and a complete understanding of the bluefin tuna reproductive schedule. Sampling that includes fish smaller than 185 cm CFL should be included to determine the lower size/age limit of this maturity curve.
Our data did not allow us to find a significant link between body condition and reproductive state but condition should not be ruled out as a possible factor for determining differences in migration routes or reproductive status. We are currently developing a more precise index of condition based on % lipid in muscle tissue to provide a more accurate measure of energetic state. Future work will include muscle tissue biopsies and/or blood samples taken from electronically tagged individuals which could prove useful in determining whether sex, (Susca et al. 2000; Heppell and Sullivan 2000) and/or lipid content of muscle correlates with migratory and reproductive behavior or spatial range as has been found in herring (Rajasilta 1992) and European eels (Larsson et al.1990)
May 8, 2007- Cheryl Harary presents research at UNH Undergraduate Research Conference
Undergraduate researcher at the Large Pelagics Research Lab Cheryl Harary recently presented her findings at the Undergraduate Research Conference. Her project is entitled "In Vitro Analysis of Digestive Processes in Atlantic Bluefin Tuna". Cheryl has been working with the lab for two years, primarily on other graduate student and PI work. During her senior year, she expressed a desire to try some independent research, and began working with Molly Lutcavage and graduate student John Logan on a digestion project, a project that compliments the stable isotope and stomach contents analysis currently going on in the lab. She simulated digestive conditions at different pHs and temperatures as well as simulating movement. She digested whole prey, squid beaks and otoliths in all combinations of factors. For more information, contact John Logan.
Cheryl graduated from UNH spring 2007. We wish her well in all of her future endeavors.
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