For those of us old enough to remember (or young enough, depending on your age), the word SMURF most likely evokes memories of little blue woodland figures brought to you by Saturday morning programming. But to members of the ODFW Oregon Marine Reserves team, SMURFs are something radically different: Standard Monitoring Units for the Recruitment of Fishes.
Which is fancy terminology for what basically amounts to a floating bunch of plastic encased by even more mesh plastic. However, to a juvenile fish, these artificial refuges offer a safe haven in the desert that is the open ocean.
Upon deployment in nearshore waters, SMURFs are left to soak for two weeks before being retrieved, during which time they are colonized by the young of a variety of fishes (in our case, mainly rockfish and cabezon). The SMURF is then enveloped in a large net, dragged onto the boat, and unceremoniously shaken against the deck to dislodge any creatures stuck within the inner crevices of plastic. Back in the lab, each fish is meticulously measured and dropped into individually labeled baggies destined for the freezer. On a good day of SMURFing, the total haul can tally in the hundreds.
Researchers with the marine reserves program and Hatfield Marine Science Center use these sample collections to piece together various aspects of the early life histories of fish. One process known as recruitment, which is defined by NOAA to be the “time when a young fish enters a fishery or enters a specific habitat such as a juvenile or adult habitat,” is of particular interest. Many larval fish species, rockfish and cabezon included, spend a portion of their time in a pelagic phase, in which they are subject to surface currents that carry them offshore. As they continue to grow, they make their way back to nearshore waters and become recruits of the more mature populations. SMURFs intercept these transients, providing valuable information on the understudied temporal and spatial links between larval and adult stages.
One of the more notable observations on our SMURFing expeditions this year has been the presence of an abnormally large size class of juvenile cabezon.
A quick discussion with colleagues back in the office has led to a few conceivable conjectures, all regarding recent El Niño conditions (Earth is currently experiencing neutral conditions, but a La Niña event is scheduled to be in place later this year). The occurrence of an El Niño leads to weakened winds that would otherwise normally push surface waters westwards across the Pacific. This in turn causes depressed upwelling of nutrient-rich cold water along the western coast of the Americas; the opposite effect is seen during La Niña conditions. Past SMURF research has shown that the periods of surface current relaxation and reinforcement associated with these climatological events favors recruitment of certain rockfish species over others, which could indicate similar successful outcomes for other larval fish species.
One of our personal theories is that the slower currents induced by the El Niño equate to a longer time spent offshore, and thus an extended period of growth before being brought back nearshore. An interesting way to test this would be to observe the temporal abundance of recruits – later arrival times than normal could lend support for this theory. Another guess is that the warmer surface water increases the metabolism of the larvae and accelerates their growth prior to recruitment. Perhaps it’s a combination of the two, or possibly something entirely different.
A cursory online search of research literature yielded little insight, although I did find this lone paragraph on cabezon and El Niño conditions from a 2009 NOAA assessment:
“The recruitment patterns…suggest a possible link between environmental forcing and population dynamics. Specifically, strong ENSO conditions…may be a pre-cursor to significant recruitment events.”
Just to be clear, it certainly is possible that the data is simply coincidental. But based on our own observations and the one above, the potential correlation of increased cabezon size with El Niño is a plausible hypothesis to be sure, and one that requires further investigation to tease apart the true dynamics of the situation. For now, though, the phenomenon remains a mystery as cryptic as the fish itself.