A Southern California Icon Surfs North: Local Ecotype of California Grunion, Leuresthes tenuis (Atherinopsidae), Revealed by Multiple Approaches during Temporary Habitat Expansion into San Francisco Bay
California Grunion Leuresthes tenuis, a beach spawning marine teleost, was described from a specimen purchased at a San Francisco market in 1859, but not subsequently seen there for 140 years. From 2001–2007, L. tenuis spawned on beaches in San Francisco Bay (SFB, 37°45′N, 122°15′W) at multiple locations, but disappeared after 2008. In 2005, L. tenuis started spawning for the first time on a beach in Tomales Bay (TB, 38°14′N, 122°58′W), 64 km north of SFB, but this population disappeared after 2009. Adult size, clutch volumes, and egg diameters of L. tenuis in SFB were consistently smaller than L. tenuis from southern California, though the population was not genetically distinct. Population size structure suggests few L. tenuis survived more than one year in these northern bays, rather than the two or three years expected in the typical southern California habitat. Climate change models predict conditions supporting poleward expansion of ranges of marine organisms, but colonization of northern habitats by this beach spawning species resulted in significant phenotypic changes including smaller size, shorter life span, and reduced reproductive output. The multiple environmental challenges and rapid extirpation of two disjunct colonization events indicate this species will require repeated events for habitat expansion to succeed.
Numbers of L. tenuis among the dropped fishes at Alameda Least Tern Colony, and as caught in monthly trawl surveys by California Department of Fish and Wildlife in San Francisco Bay from 2001 through 2008. Trawls were done year-round. Dropped fishes were counted only when birds were present, from May to August.
Map of SFB showing confirmed locations of spawning runs of L. tenuis. A = Crissy Field; B = Foster City; C = Seaplane Lagoon, and also Alameda Least Tern Colony; D = Albany Bulb; E = Emeryville; F = Middle Harbor, Shoreline Park, Oakland; G = Crown Memorial State Beach; H = San Leandro. Leuresthes tenuis were collected at least once at all trawl stations within south and central SFB, and the two most western stations in San Pablo Bay, all areas below the heavy bar.
Walker Scale ratings (see text) for 82 observations for spawning runs of L. tenuis on beaches in San Francisco Bay (SFB) and Tomales Bay (TB) from 2005 to 2008. If multiple nights were observed during a spawning period, the largest sighting is reported. No runs were seen in 2008 or subsequently, despite repeated attempts, although two L. tenuis were observed at TB in 2009.
Frequency distribution of sizes (SL) of dropped L. tenuis (n = 308) from the Alameda Least Tern Colony and trawled L. tenuis (n = 686) from CDFW surveys. Fish smaller than 40 mm would have passed through the trawls without being caught. For comparison, the size of mature L. tenuis(n = 277) caught during spawning runs in 2005 and 2006 in southern California are shown.
Leuresthes tenuis that first appeared in TB in 2005 are not different from L. tenuis from southern California (SoCal) in length (mean±SD). The SL of L. tenuis from SFB and TB in later years were not significantly different from each other whether collected by trawl, dropped fish, or during spawning runs, but SFB specimens were significantly smaller than L. tenuis from southern California. Different letters indicate significant differences.
Larger L. tenuis were rarely seen outside of the summer spawning season. July shows the greatest variability as young-of-the-year are large enough to appear along with spawning adults. The remainder of the year shows growth of the juveniles to maturity.
(A) Clutch volumes for L. tenuis spawning on beaches in SFB were significantly smaller than clutches from southern California. (B) Egg diameters (mean±SD) of eggs of L. tenuis collected. SFB eggs were significantly smaller than those from southern California.
Contributor Notes
Associate Editor: J. F. Schaefer.