The great stocks of sardines and anchovies, and other small pelagic fishes, account for about one third of the world's yield of marine fish and are of key economic importance in many nations. Production of these stocks depends upon a delicate balance of physical ocean processes. When environmental conditions in the ocean are optimal, great year classes result and populations grow rapidly. The optimal environmental window for small pelagic fish depends upon a triad of physical factors: enrichment processes that lead to the production of the zooplankton upon which the young stages depend for food; concentration processes that aggregate foods and thereby increase their availability to growing larvae; and retention processes that keep the young in their favored nursery habitat. Without a doubt, global heating will alter this triad of physical processes since all are functions of atmospheric forcing, ocean circulation, and fresh water inflow--all of which are expected to be altered by climate change.
Many present day populations of small pelagic fishes display a complex pattern of vital rates indicating adaptation of subpopulations to local habitat conditions. Some subpopulations are tiny with maximum biomass less than 20,000 mt while others reach millions of metric tons. These subpopulations experience different environmental conditions and are natural models of how marine populations react to environmental change. No group of marine populations is better suited for examining the linkage between physical forcing and population dynamics and structure than the small pelagics because of their world wide distribution, long time series in abundance, presence in the paleoecological records, and the wealth of information on their ecology and dynamics. Finally, the dominant small pelagic fish species shifts over decades in most systems. Changes in species dominance is due presumably to subtle changes in the suitability of the habitat. Shifts in dominants often occur nearly simultaneously in different regions of the world's oceans suggesting that atmospheric teleconnections may be important.
A common theme of the meeting was that significant advances could be made by comparing zooplankton production systems and the vital rates of small pelagic fishes in different ecosystems. Participants felt that within a zooplankton assemblage, a limited number of key species may exist that show strong links to larval and adult fishes. Advances in understanding will require recognition and description of these links, and perhaps monitoring or detailed examination of trophic and food web relations. The concept of an optimal environmental window for wind stress and other environmental variables was given considerable attention at the meeting, not only in the context of fish life history, but in the context of zooplankton production as well. Key in this regard is the extent that the optimal windows for zooplankton overlap those of small pelagic fish.
Modeling functions as a research tool that provides a framework for hypothesis testing by putting disparate field measurements into a common framework. It is needed to summarize accumulated information, provide the linkage between historical data sets, retrospective studies, and field process studies, and develop predictions regarding the effects of climate change. A variety of models are needed, ranging from energy budget models of key species to physical models of regional circulation and mixing dynamics. Especially valuable are models that bridge the interface between biology and physics.
Temporally continuous monitoring of ocean dynamics is essential for SPACC process studies because environmental events throughout the year could be critical to the survival and growth of young fishes and production of their zooplankton forage. Intermittent information provided by research cruises must be temporally and spatially supplemented using satellite imagery of sea surface temperature, cloud cover and ocean color; coastal station records of wind, sea level, and salinity; and, low-cost subsurface observations.
The spawning habitat of most small pelagic fishes is bounded by a front or pycnocline between warm and cool water or between saline and fresher water. This perhaps was the only common denominator among the diverse array of spawning habitats reviewed at the meeting. These boundaries may provide water column stability, nutrient enrichment or concentration of food particles at the interface. The mechanism by which these boundaries provide benefits to the survival of the early stages of pelagic fishes need identification. To determine the role of these boundaries requires contemporaneous measurements of flow fields and larval distributions.
Determining how the local habitat of small pelagic fishes is affected by climatic variability was central to the meeting. In particular, to determine whether the dynamics of populations are due to local events or large scale climatic forcing. A consensus developed that retrospective analysis of the sedimentary records of anaerobic marine deposits could provide the long time scale data needed to characterize climate variability. In addition to providing a history of population abundance of small pelagic fishes (inferred from fish scale abundance in sediments), ecosystem histories could be developed from the sedimented remains of plants and other animals, and from chemical components. Interpretation of such paleoecological records is greatly improved by accurate description and understanding of the process of sedimentation at a site (e.g., sediment trapping). The scales of physical forcing can be studied using historical time series of biological and physical data. Evidence is mounting that ecosystems supporting small pelagic fish populations undergo productivity changes of decadal frequencies which are expressed, inter alia, by "regime shifts" of clupeoid populations. The causal mechanisms need identification, perhaps using new methods for analysis of time series of phytoplankton, zooplankton, fish, and physical data.
SPACC is in a position to address the systematics and population genetics of small pelagic fishes in the world's oceans using modern molecular methods. For many of the small pelagics, it is unclear where species and population boundaries begin and end. Moreover, climate change may rapidly influence the genetic and demographic structure of small pelagic fish populations because they are relatively short-lived and feed at the top of a short but unstable plankton-based food chain. Modern molecular methods may be useful in examining genetic diversity between and within populations, and perhaps also through time (using scale DNA).
GLOBEC-INT leadership in SPACC could assist in program development and technology transfer. GLOBEC-INT could aid program development in a region, or country, by providing support for regional workshops where participants from different universities, institutes, and fishery agencies could be encouraged to cooperate to develop and implement a SPACC study. A TEMA (training, education, and mutual assistance) component must be developed within SPACC to provide training for making routine measurements, using advanced oceanographic equipment, and interpreting data. Cited examples include training in: 1) modern methods of measuring zooplankton production; 2) determining back-calculated daily larval growth using otoliths of juvenile fish; 3) using advanced gear such as undulating samplers; and, 4) interpreting the data obtained from ADCPs and satellites. (This article abstracted from documents provided by John Hunter of the NMFS--La Jolla. Dr. Hunter is a former member of the Steering Committee of U.S. GLOBEC, a current member of the organizing committee of GLOBEC International, and co-chaired the SPACC workshop.)
