Science1 publisher2 min readPublished
Bermuda's lobster catches track a plankton food-chain index more closely than sea temperature
Arizona State-led researchers built a plankton food-chain index that explains Bermuda's spiny lobster catch rates better than four simpler predictors. Officials at Bermuda's environment department say it can warn them of trouble in the larval stage before the catch shows it.
The Scientist · Science desk
What happened
- Spiny lobster larvae drift six to nine months among open-ocean plankton before settling into mangroves, seagrass beds or coral reefs.
- The index merges the fishery's catch-per-unit-effort records with the Bermuda Atlantic Time-series Study and satellite readings of ocean photosynthesis.
- The authors found the regional ocean has been warming while satellite-measured primary production has fallen.
- Shifts in the zooplankton community suggest energy is passing up the food chain less efficiently than before.
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Why it matters
- decision Bermuda's managers now have a basis for adjusting lobster management before a change in the early life stages turns up as lower catches.
- constraint Young lobsters reaching Bermuda depend on conditions across the wider North Atlantic and Caribbean, so local rules can prepare for a poor larval year but cannot prevent one.
- precedent Fisheries elsewhere that keep catch records alongside long ocean time series and satellite data have the inputs to try the same index, and the authors plan to test it there next.
I'd look first at what the index was compared against. The team tested it against four simpler explanations of the catch record: sea temperature, primary production on its own, previous catch levels and long-term average conditions [8][1]. Those are the obvious null models. Previous catch is, I think, the hardest of the four to beat, because it asks whether the index knows anything the fishery's own recent history does not. The index explained catch rates better than all four, and forecasts built on it outperformed them too [8][9].
What the index predicts is catch per unit effort from Bermuda's commercial fishery [3]. The link to larvae is an inference. The authors reason that the food-chain changes they found "may mean" less nutrition for larvae during their months adrift, and for juveniles and other animals after settlement [7]. The hedge is theirs, and it fits the evidence. An index that tracks catch is consistent with a food-limited larval stage, but showing that fewer larvae are actually arriving would take counts of settling young. The published account does not mention such counts, and it does not give the size of the index's advantage or how far ahead its forecasts reach [9].
Bermuda's regulators want it for early warning. "Most fishery monitoring is based on looking at changes in the catch after those changes have happened," said Joanna Pitt of the Department of Environment and Natural Resources [10]. "The major benefit of this index is that it can give us some advance warning of changes that are affecting the early life stages of the spiny lobster, so that we can be prepared to adjust our management approaches" [10]. The authors say the index is meant to complement existing monitoring and management tools and to improve forecasts [11].
For that job, the inputs matter as much as the fit. A regulator needs a forecast it can rerun each season. Every input here is a record that Bermuda's fishery agency, the time-series program and satellite missions already keep, and together they span decades [3][12]. The paper's title calls it "a simple trophic transfer index" [4].
"Although this ecological index has been developed for the Bermuda fishery, its underlying principle could be applied in any ocean region: how much energy is produced, and how much of that reaches our fisheries," said Leocadio Blanco-Bercial, the lead author and an associate professor at Arizona State University's School of Ocean Futures [13][15].
What to watch
- Results from testing the index in other regions and fisheries with comparable catch, time-series and satellite records.
- Direct counts of settling spiny lobster larvae at Bermuda that confirm or contradict the food-limitation explanation.
- Whether Bermuda's Department of Environment and Natural Resources changes lobster management on the strength of an index forecast.