We measure fish recruitment in two ways: Standardized Monitoring Units for Recruitment of Fishes (SMURFs) and visual surveys as part of our kelp forest community monitoring program. In addition to regular recruitment monitoring, we perform process oriented studies to investigate specific aspects of recruitment and it’s affect on populations.
PISCO researchers have discovered links between recruitment of fishes and climate-related oceanographic cycles such as El Niño, La Niña and coastal upwelling and relaxation that have dramatic effects on nearshore ocean circulation, nutrients, and water temperatures. Our research shows that the young of some species of rockfishes that dwell in kelp forests add to the adult populations in greater numbers during El Niño than during La Niña, while the young of other rockfishes display the opposite trend. The pattern appears to arise from the fishes’ differing life histories and from variation in ocean circulation. While that pattern is strong in the Monterey region, it is not observed in the Santa Barbara channel region where rockfishes of all groups tend to settle in the same years. However, in the Santa Barbara channel region, strong settlement years for the cold-water rockfishes tend to be low settlement years for the warm water, subtropical fishes such as kelp bass. The links among climate, oceanography, and the replenishment of fish populations can strongly affect kelp forest ecosystems. This research provides information that is integral to understanding and managing the system as a whole.
A study based on 7 years of data on larval supply and adult survivorship of a reef fish (kelp bass), and giant kelp abundance for 13 populations spread over 200 km in the Santa Barbara Channel has shown that at the small, within-site scale, both recruitment and adult survivorship of kelp bass were density dependent and positively related to kelp abundance. At the larger, among-site scale, the spatial pattern of adult kelp bass abundance was predicted well by the pattern of kelp bass larval supply, but there was a consistent negative spatial relationship between kelp abundance and kelp bass larval supply despite the positive effects of kelp on kelp bass at the smaller spatial scale. This large-scale negative relationship was likely a product of a channel-wide spatial mismatch between oceanographic conditions that favor kelp survival and those that concentrate and distribute fish larvae. These results generally support the recruit– adult hypothesis: kelp bass populations are limited by recruitment at low recruit densities but by density-dependent competition for food resources and/or predator refuges at high recruit densities.
Research by graduate student Darren Johnson investigated the effects of habitat complexity on mortality of blue rockfish, Sebastes mystinus, within nearshore reefs off central California (Ecology, 2007). Increased habitat complexity was associated with a reduction in both density-independent and density-dependent mortality. At low levels of habitat complexity, limitation analysis revealed that mortality was strong and recruitment had relatively little influence on population size. However, as habitat complexity increased, recruitment became more important. At the highest levels of habitat complexity, limitation by recruitment was substantial, although density-dependent mortality was ultimately the largest constraint on population size. In high complexity habitats, population dynamics may strongly reflect variation in recruitment even though fluctuations may be dampened by density-dependent mortality. These findings suggest that the relative importance of pre- vs. post-settlement factors may be determined by quantifiable habitat features, rather than ambient recruitment level alone. Because the magnitude of recruitment fluctuations can affect species coexistence and the persistence of populations, habitat-driven changes in population dynamics may have important consequences for both community structure and population viability.