Downloading menhaden catch on the deck of a ship. (Photo:NOAA)
Climate cycle could determine the reproductive success of menhaden
Friday, February 19, 2016, 22:30 (GMT + 9)
Scientists have long puzzled over what drives the reproductive success of Atlantic menhaden, a tiny but critical East Coast fish.
A new study, published in the ICES Journal of Marine Science and supported by the Lenfest Ocean Program, provides a partial answer: An oceanic climate cycle known as the Atlantic Multidecadal Oscillation, or AMO.
The finding is likely to be useful in improving the way scientists assess the species and the way managers set catch limits.
Menhaden is a small, oily fish that provides food for striped bass, bluefish, and several other species, as well as bait for fishermen.
It also is the target of the largest fishery on the East Coast, which is centred in Virginia and catches menhaden for use in nutritional supplements, animal feed, and fertilizer.
Setting catch limits has proved challenging, in part because no one has been able to say what drives recruitment—a technical term for how many young fish are produced. Recruitment largely determines how much fishing a population can sustain.
Tom Miller of the University of Maryland, and one of the authors of the study, explained that it is very difficult to understand and manage this stock.
To address this, the researchers used a statistical model designed for disorderly data like those on the abundance of young menhaden. They considered 16 factors that might be driving that abundance, including climatic cycles, intensity of fishing, temperature, salinity, and predator abundance.
This way, they found that the most important driver of recruitment is the AMO. In this natural cycle, average sea surface temperature shifts by about one degree Fahrenheit (0.56 degrees Celsius) every 20 to 40 years across most of the north Atlantic.
The effect of the AMO on menhaden is different in different places.
South of Cape May, New Jersey, the warm phase of the AMO, which has been in effect since the 1990s, is generally associated with menhaden recruitment being lower than average. North of Cape May, the warm phase is associated with higher than average recruitment. It is not yet clear why the AMO has this effect, but it could be exerting its influence through water temperatures or through the currents needed to transport menhaden eggs and larvae into suitable nursery habitat.
The finding suggests that current management, which does not directly consider environmental drivers, could result in unsustainable catch limits in bad recruitment years and unnecessarily stringent limits in good years.
“There is a surprisingly clear climatic effect,” Andre Buchheister, the study’s lead author and a fisheries biologist at Humboldt State University in California noted.
“We see a number of ways to incorporate this into the process for setting catch limits that match the rate at which the stock can replace itself. For example, the AMO could simply be used as an indicator for when to reduce target fishing mortality,” he added.
In addition, two other findings relevant to the resource management were spotted.
On the one hand, the study did not detect a correlation between recruitment and the number of eggs produced by menhaden. This would indicate that the current use of egg count as the main indicator of how many fish will be produced is insufficient. However, the research suggests that the size and/or age structure of the adult population may be an important factor in understanding the extreme variation in how many young fish are produced each year.
The study also did not reveal a negative effect of fisheries landings on recruitment. Still, it would be wrong to conclude that high catches are therefore sustainable.