Variability in abundance of adult stages in marine fishes is generally thought to be a function of processes operating in the larval or early juvenile stages. Better understanding of these processes has tremendous value in predicting the abundance of an age cohort later in life (year class strength), and for evaluating the potential impact of both natural and human-induced environmental changes on fish populations. Despite extensive research efforts in this field in recent decades, definitive linkages between environmental patterns and larval/juvenile survival remain elusive. Complex interactions of spatial and temporal patterns in habitat quality, physical conditions, and the community structure of interacting species presumably contribute to the difficulty in resolving discrete causal relationships.
The continuing threatened status of West Coast salmonid populations and the currently developing groundfish crisis warrant more intensive research into the array of factors driving early survival. Current projects being conducted by the Early Life History team focus on larval quality and growth rates as indicators of individual fitness. This individual variability provides the template on which mortality acts. Our research attempts to understand both the long term evolutionary selection pressures that maintain individual variability and the short term effects of individual differences on survival under specific environmental conditions.
Current Research Projects
I. Maternal Effects on Larval Quality in Rockfish
Rockfish populations along the west coast of the U.S. have recently undergone severe declines, prompting emergency management measures that include prohibiting fishing along extensive areas of the continental shelf. Most rockfish are long-lived, with maximum ages ranging from around 30 to over 100 years depending on the species. Fishing tends to remove the larger and older members of the population, resulting in a greatly reduced spread of ages. We are currently investigating the impact of this demographic shift in age structure on the long-term sustainability of rockfish populations. Our primary hypothesis is that larval quality is a function of female age, with older females producing progeny with a greater likelihood of survival than larvae from younger females. Rockfish are livebearers with internal fertilization. Eggs hatch within the female's ovaries and develop up to feeding stage. At parturition, or release of the larvae by the females, they retain some yolk and an oil globule rich in lipids. We believe the energy reserves provided in the oil globule are key to a larva's capacity to survive in the tenuous planktonic environment. We are collaborating with Steve Berkeley, a researcher at UCSC, whose prior experiments demonstrated that older black rockfish females provision their larvae with larger oil globules compared to younger females. Consequently, larvae from older mothers have higher survival and faster growth rates. There are important implications of these results for fisheries management, since current regulatory options typically provide little protection for older age classes.
To test the generality of a maternal effect across the broad range of rockfish occurring in the Northeast Pacific (~72 species), we are collecting pregnant females in the field, holding them in the laboratory until parturition, and monitoring subsequent growth and survival of larvae. Photographs taken at progressive stages allow us to measure the oil globule and its depletion during development, along with growth of the larvae [see pictures]. The ages of the mothers are determined from their otoliths, and larval performance is compared with female age to evaluate age-related maternal effects. Future studies will expand to evaluate growth rates of larvae and early juveniles in the field, examine selective processes that influence which individuals survive the early stages, and determine the role of environmental variability in shaping survival patterns.
II. Life History Trajectories in Steelhead
Steelhead (anadromous rainbow trout) in central California are currently listed as threatened under the Endangered Species Act. Our objective with this research is to provide ecological information useful in planning for recovery and management of local populations. Steelhead exhibit a remarkable plasticity of life histories. They can remain in their natal stream as immature juveniles for 1-4 years, migrate to the ocean and stay there 1-4 years before returning to spawn, mature as small parr and then still go to sea and come back, or stay in fresh water for life (i.e., become a rainbow trout). Unlike most Pacific salmon species, steelhead often survive after spawning and may spawn again after another year or two of ocean residence. This diversity of life history strategies presumably helps ensure continued success of a population in fluctuating environments, but such diversity may be reduced or modified by a variety of natural and human-induced habitat changes. The factors that determine which life history pathway an individual will follow are currently unknown, but are thought to include growth rate and condition (i.e., lipid storage), which are in turn affected by habitat quality (prey availability, temperature) and fish density.
In this project we are monitoring growth of juvenile steelhead in hatchery and natural populations during their first summer growing season. In the fall, fish are implanted with PIT (passive integrated transponder) tags, which allow identification of individuals. Growth trajectories are tracked on an individual basis into the following spring, when seawater adaptation (smoltification) is assessed using an enzyme analysis of physiological changes in the gills. Continued tracking of PIT-tagged juveniles that remain in streams for a second year and adults upon their return to natal streams after ocean residence will provide further information on life history consequences of variability in early juvenile growth. These field studies are complemented with laboratory experiments using age-0 steelhead. We are testing specific effects of food availability, temperature, photoperiod, and other environmental factors on individual growth and behavior (activity levels, social interactions, responses to predators) during the winter. Smoltification of each fish will be analyzed in the spring and compared with indices of growth and behavior. Future studies will include incorporation of the field and laboratory results into a life history model allowing prediction of the consequences of environmental change.