Rocky Reef Fish Connectivity: Patterns, Processes and Population Dynamics
The extent to which marine populations are connected by dispersing larvae and the ramifications of this connectivity for population dynamics was investigated for the temperate damselfish, Hypsypops rubicundus, in San Diego County, USA. Surveys identified six source populations for this species: Carlsbad, Cardiff, Torrey Pines, La Jolla, Mission Point, and Zuniga Point. Three of these reefs are within or adjacent to existing marine protected areas. Trace elemental fingerprinting was used to quantify the connectivity of populations in 2008-2009. High-resolution sampling over a protracted spawning season revealed that elemental fingerprints of reefs earlier in the spawning season became indistinguishable from other reefs later in the spawning season, resulting in inaccurate assignment of natal origin of post-dispersal fish. When natal origins of fish were assessed using appropriately binned data, one reef, La Jolla, emerged as the predominant source population, supplying itself and three other reefs with recruits. The northernmost reef, Cardiff was a “pure” sink, in that it unilaterally imported fish. Dispersal trajectories predominantly were in a northerly direction, but sporadic southerly dispersal was documented, corresponding to empirically measured current reversals. On intra-annual time scales this network of reefs resembles a source-sink metapopulation, but over annual time scales it functions as an open metapopulation with a well-mixed larval pool. To assess the demographic significance of observed connectivity patterns, empirically parameterized, stage-based matrix models were coupled with connectivity matrices. Elasticity analyses suggest inter-reef connectivity acts primarily to regulate which vital rates are demographically most significant; at low levels of connectivity adult survivorship has the greatest influence on population growth rate; at high levels of connectivity, juvenile growth is most influential. Quantitative metrics of sources and sinks were developed and node deletion experiments conducted to better characterize reef connectivity within the metapopulation. La Jolla was identified as the most valuable reef within the metapopulation in terms of connectivity; it may regulate how populations of fish at other reefs persist over time, and as such should be a conservation priority. New knowledge of the magnitude, directionality and variability of connectivity, and its roles in regulating local and metapopulation dynamics will aid local marine conservation efforts.