FRISCO — Biologists looking at 40 years of fisheries data from Puget Sound have documented a dramatic shift in marine species. Key fish in the food chain, like herring and smelt, have declined, while the number of jellyfish has increased exponentially, to the detriment of the marine ecosystem.
“On land people see the changes that come with human population increases, but underwater the changes are much harder to discern,” said Correigh Greene, with NOAA Fisheries’ Northwest Fisheries Science Center. “What this tells us is that when you look over time, you can see that the underwater landscape of Puget Sound is changing too,” said Greene, lead author of the new study published in Marine Ecology Progress Series.
The long-term study enabled the scientists to track parallel trends of rising human population and declining forage fish. The findings suggest a strong link between human influences such as pollution and development and the loss of species that were once common in Puget Sound. In particular, the rise of jellyfish blooms may divert energy away from highly-productive forage species that provide food for larger fish and predators such as salmon, seabirds and marine mammals.
The study was based partly on data from trawl surveys of fish species in Puget Sound in the 1970s and 80s. In some cases, the scientists salvaged written records headed for the recycling bin. Then they compared those early results to similar results from their own surveys in 2003 and 2011 to detect changes in the abundance and distribution of forage fish species and jellyfish.
The Puget Sound Partnership cites Pacific herring as an indicator of ecosystem health in Puget Sound.
Jellyfish blooms grew as the area’s human population exploded, with jellyfish-dominated catches jumping three to nine-fold in the same sub-basins where herring and smelt declined. In some cases more than nine of every 10 tows of trawl survey nets in recent years brought in catches dominated by jellyfish.
The shifts suggest that the same disturbed conditions linked to declines in herring and smelt may benefit more opportunistic jellyfish, which then further pressure forage fish by competing with them for food or even consuming their eggs and larvae. But jellyfish offer far less nutrition than the declining forage fish, reducing the food and energy available to species higher on the food chain.
Harvest of forage fish may open new opportunities for jellyfish by reducing competition from other species and human-driven changes in habitat may reduce the productivity of forage fish, scientists suggest. Polluted runoff may also shift prey towards types that jellyfish favor.
The research may also help resolve the mystery of why juvenile salmon survival has declined sharply in Puget Sound. While forage fish may compete with salmon in some circumstances, they also serve as prey for salmon and can help absorb some pressure from predators such as seals that might otherwise consume young salmon. The parallel declines of forage fish and juvenile salmon survival suggests the loss of forage fish may also affect salmon.
“We still have to pin down mechanisms and causes, but there is clearly a compelling pattern of change in Puget Sound food webs that may be linked to human influence,” said Casey Rice, a research fish biologist at the NWFSC and co-author of the new paper. “These results are a potent reminder of just how important such field studies are in detecting, diagnosing and managing impacts on natural resources.”
Declines in forage fish were most closely related to human population density, while commercial fishing and climate were less important factors, the research found.
The results may help guide habitat protection and restoration by focusing it on less-disturbed parts of Puget Sound that remain important habitat for forage fish, the researchers concluded. The findings also suggest that efforts to rebuild forage fish populations should take into account human pressures that may be driving unrecognized changes in marine ecosystems.
“We were fortunate to find the long-term data that allowed us to detect these changes in Puget Sound,” said Lauren Kuehne, a research scientist at the University of Washington and co-author of the research. “This really demonstrates the value of data that may have been collected many years ago for different reasons but helps us see the changes that are affecting ecosystems today.”
–From Summit County Voice