Now, researchers at the University of California, Santa Cruz, are working out the details of the oceanographic conditions that determine how and when juvenile rockfish return to the kelp beds. New instruments that provide detailed information about distinct layers and movements of ocean waters along the coast are playing a key role in this effort.
"Only now do we have the technology to see the fine-scale thermal and density structure of the ocean and make these connections between specific oceanographic events and the biology of fish and other organisms," said Margaret McManus, an assistant professor of ocean sciences at UCSC.
UCSC graduate student Arnold Ammann found that juvenile rockfish return to their nearshore habitats in pulses, the timing of which depends on the species. Linking these patterns with oceanographic observations, McManus and her collaborators found that some species are brought in with the periodic upwelling of cold, deep water, while others come in when the upwelling subsides. In addition, McManus has found that fish larvae and other plankton are often concentrated in previously undetected thin layers of ocean water that extend for miles and persist for days at a time.
"We have found concentrations of organisms in these layers five times higher than in surrounding waters," McManus said.
McManus will present the group's findings in a symposium, "Opening the Black Box: Understanding Ecosystem Dynamics in Coastal Oceans," at the annual meeting of the American Association for the Advancement of Science (AAAS) in Denver.
McManus and postdoctoral researcher Curt Storlazzi collaborated with Ammann, currently a research fisheries biologist at the National Marine Fisheries Service (NMFS) laboratory in Santa Cruz, and UCSC marine ecologists Mark Carr and Peter Raimondi. Their ongoing research is part of the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO), a multi-institutional collaboration funded by the David and Lucile Packard Foundation. Carr and Raimondi are the lead investigators for PISCO at UCSC.
Scientists are keen to understand how rockfish populations are replenished, because the populations of many West Coast species have declined dramatically in recent years, Carr said.
"The delivery of the young fish back into the kelp beds is what replenishes rockfish populations," he said. "One of the key questions for marine ecologists and fisheries biologists is to understand what determines year-to-year variation, as well as geographic variation, in the replenishment of these fish populations."
Surveys conducted by Carr in the late 1990s found striking year-to-year variations in the numbers of juveniles "recruited" into different rockfish populations. Rockfish species responded differently to the changes in oceanographic conditions associated with the 1998 El Niño and the subsequent La Niña in 1999. The species fell into two groups: one whose juveniles showed up in the kelp beds in large numbers during El Niño but did poorly during La Niña, and another group that had the opposite response, recruiting poorly during El Niño and doing well during La Niña.
"Those patterns suggested that oceanographic conditions influence rockfish species differently and determine the year-to-year variation in recruitment," Carr said.
To follow up on those findings, the researchers designed a coordinated study in which McManus and Storlazzi obtained oceanographic data with an array of new instruments while Ammann collected detailed data on recruitment of juvenile rockfish into nearshore populations. Ammann set out SMURFs (Standard Monitoring Units for Recruitment of Fishes)--mesh tubes filled with a plastic material that mimics the juveniles' preferred habitat--at sites in Monterey Bay. Every two to three days, Ammann would collect fish from the SMURFs and identify and count them. Meanwhile, the group monitored currents, temperatures, and a range of other conditions in the bay.
The results showed that juvenile rockfish show up in pulses during the year that correspond to different phases in the periodic upwelling of deep water along the coast. Upwelling occurs during the spring and summer when winds from the northwest blow warm surface waters offshore and stir up deep, cold water nearshore. Every couple of weeks or so, the winds shift and the upwelling subsides, allowing warm water to move back to the coastline--this is known as relaxation.
It turns out that the rockfish species that recruited large numbers of juveniles into the kelp beds during El Niño also showed high recruitment during relaxation events. The species that recruited well during La Niña, meanwhile, showed high recruitment during upwelling events. This makes sense because during an El Niño upwelling is suppressed by an influx of warm tropical water along the California coast. Conversely, during La Niña the upwelling of cold water is especially intense.
"We've found there are two groups of rockfish, one that depends on warm water events and another that depends on cold water events for the recruitment of juveniles," Ammann said.
Surveys by NMFS biologists have shed some light on the offshore phase of the rockfish life cycle. When they are released, rockfish larvae are very small and their ability to swim is limited, so they are at the mercy of the currents. As they grow and are better able to swim, the species that recruit during upwelling move down into deeper water, whereas the species that recruit during relaxation events stay near the surface.
"The ones that recruit during upwelling basically make a big vertical loop during that phase of their life cycle," Carr said.
Another difference between the two groups is that the larvae of those recruited during upwelling tend to spend more time in the dispersal phase than those that recruit during relaxation events (several months versus several weeks). The species that recruit during upwelling events include black rockfish, blue rockfish, olive rockfish, and yellowtail rockfish. The species that recruit during relaxation events include gopher rockfish, kelp rockfish, and black and yellow rockfish.
Another link between physical oceanography and the distribution of young rockfish that the PISCO researchers are exploring is the concentration of some fish larvae in thin layers. McManus and collaborators Percy Donaghay of the University of Rhode Island, Van Holliday of BAE Systems in San Diego, and others have been investigating the phenomenon of thin layers in coastal waters for more than eight years using new instruments the team has developed.
Thin layers are coherent patches of plankton that range in thickness from about 4 inches to 10 feet, much thinner than could be detected by traditional sampling methods using towed nets and bottles. Monterey Bay shows the strongest layering of any site the researchers have studied, McManus said.
The layering occurs during upwelling conditions, when the water in the bay flows in a large, counterclockwise gyre and becomes stratified into layers. When the upwelling subsides, the layers break down and the organisms that had been concentrated in a thin layer become mixed throughout the water column. McManus believes that the larvae and juveniles of the rockfish species that recruit during relaxation events are associated with the thin layers.
"We think that the fish larvae are in the thin layers, because that's where all the food is, and during relaxation events they get mixed out, and that's when recruitment occurs," McManus said.
Two key instruments used in the thin layer studies are an acoustic profiler and an optical profiler, both of which McManus helped develop. The combination of these instruments provides a detailed profile of the water column, from near the ocean bottom to the surface, showing not only physical conditions such as temperature and salinity, but also how much phytoplankton (algae and other plant life) and zooplankton (including fish larvae and other small animals) occur at every depth. The new instruments are accurate to within a few inches, provide continuous monitoring, and use radio transmitters to send data streaming back to researchers' offices onshore.
"I can sit in my office and see what the conditions are out in the bay at any time," McManus said.
To find out what was in the layers, PISCO divers collected samples from a location where the instruments showed a distinct layer. Initially skeptical, the divers reported that as they descended they suddenly encountered a six-foot-thick layer with drastically reduced visibility. The water samples they brought back from the layer showed three- to five-times higher concentrations of a wide range of organisms compared to the water above and below the layer. There were many types of zooplankton, including fish larvae, as well as phytoplankton, bacteria, and viruses.
The fish species in these preliminary samples were not identified, but intensive sampling of the layers, including identification of fish larvae, is planned for later this year, McManus said.
"We really want to know why these organisms are concentrated in the layers and how they are interacting," she said.
The presence of thin layers in Monterey Bay with such high concentrations of various organisms has implications beyond the replenishment of rockfish populations, McManus said. For example, monitoring for harmful algal blooms currently relies on samples collected at the surface or with nets towed behind a boat. But if toxin-producing algae are concentrated in a thin layer, they could go undetected.
Fisheries scientists, meanwhile, are especially interested in the influence of major oceanographic events such as El Niño and La Niña on the replenishment of rockfish populations. For at least some species, strong recruitment during extreme events may play a key role in maintaining populations, Carr said.
"During the last La Niña, we saw a surprisingly large recruitment of bocaccio, which is one of the more threatened species of rockfish, and we hadn't seen much recruitment of bocaccio for a couple of decades before that. So these episodic events can be very important," he said.