Toxic jellyfish are invading Europe's Mediterranean beaches, and people are suffering. Is it overfishing? Or is it...THE SWARMAs blogfish packs for vacation, I must admit I've finally picked up a copy of Frank Schatzing's THE SWARM. It's an ocean techno-thriller, where the ocean fights back against our misuse and abuse. Jellyfish start to...wait, you'll just have to read it.
In the real world, jellyfish swarms seem to be getting worse. Climate change, overfishing, or natural? You be the judge.
Meanwhile, I can't wait to get on the plane and get back into THE SWARM... Tweet
4 comments:
How about eutrophication?
Caddy (1993) suggested that gelatinous zooplankton become more abundant as systems become eutrophic or dystrophic. The increase in jellyfish is problematic for fish as they both compete with larval fish for food and consume larval fish.
Sounds like a great time to market Jelly Burgers! The nematocysts just add a little kick, kind of like pelagic hot sauce!
Then again, maybe it is overfishing:
Recent work on the Black Sea ecosystem suggests that overfishing has been the primary driver there with respect to two major (and several minor) regime shifts in that system, including one involving a "jelly takeover". Pollution and alien species introductions have also previously been blamed for these changes, but the recent work suggests that these are both secondary and that overfishing has been the primary driver of change. See summary below from Science regarding the recent original paper in the Proc. of Nat'l Academy of Sciences.
EDITORS’CHOICE
E C O LO G Y/ E V O L U T I O N
Fishing Induces Regime Change
The speed of change in ecosystems ranges from the imperceptible
to the abrupt. Rapid, nonlinear changes (referred
to as regime shifts) over time scales as short as 1 year are
by their nature difficult to study and even more difficult to
attribute to specific causes. Nevertheless, the accumulation
of data over periods of decades can provide critical
tests of mechanistic proposals.
Using time series data from fishery catches, long-term
monitoring of plankton and planktivorous fish biomass,
and oxygen concentration measurements over the past 50
years, Daskalov et al. describe two major regime shifts
and several minor ones in the Black Sea ecosystem. Predatory
fishes were heavily depleted in the 1960s, causing a cascade of effects down the food chain in the 1970s whereby
top-down consumer control was replaced by bottom-up resource control of the system, which became dominated by
planktivorous fishes. A second major shift happened in the early 1990s, when there was a population collapse of
planktivorous fishes and an outburst of an alien jellyfish Mnemiopsis leidyi. The time series data suggest overfishing
as the driver of both of these shifts, rather than pollution or the alien invasion per se. The top trophic level of predatory
fish has not recovered (and seems unlikely to), although the appearance of the jellyfish Beroe ovata, which preys
on M. leidyi, may promote the recovery of the next highest trophic layer of planktivorous fish. — AMS
Proc. Natl. Acad. Sci. U.S.A. 104, 10518 (2007).
I've looked at the Daskalov paper and I don't think that it is totally convincing that overfishing is the major cause of the problems in the Black Sea. My major issues with the paper after a quick read over are:
1.) Their pelagic populations are overly wide groupings. Specifically they lump all of the gelatinous zooplankton into a single group and all the zooplankton into a single group. After reading their discussion I don't think that was a proper move as there are a lot of internal dynamics going on within that gelatinous zooplankton group. Mnemiopsis is being consumed by Beroe and an increase in Mnemiopsis was responsible for a decline in the abundance of Aurelia (this may not have been mentioned in the current paper, but Daskalov mentions it in another of his papers).
Additionally, the zooplankton group includes at least 2 trophic levels and quite possibly 3 or more.
2.) The production of fish and zooplankton biomass isn't just an issue of quantity. Quality matters as well. Zooplankton and fish both need specific prey types to be available for them to do well. It isn't clear from the paper what exactly it is they were measuring when they gave phytoplankton and zooplankton biomass.
3.)They assume that each of the pelagic populations exist at a single trophic level. I think this is a poor assumption, particularly when many (most?) fish populations experience their heaviest mortality during the larval stage when their trophic level is closer to 2 than 3. Far more individuals die of various causes during the larval stage than from fishing when they are adults.
4.) They ignore the potential role that ctenophores could play as competitors. It would be interesting to see a plot based on their Figure 2 showing planktivorous fish and gelatinous zooplankton. It would be even better if they could show planktivorous fish vs. Beroe, Mnemiopsis and Aurelia seperately.
Just to be clear, I'm not saying that overfishing is not an issue, I'm saying that it isn't clear that it is the major issue.
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