Larval anchovy and sardine abundances between Punta Eugenia and Point Conception from 1963-2015

Anchovies and sardines are some of the most economically and ecologically important and well-studied fishes on earth but there is still uncertainty regarding how distributions and abundances change through time and space.  We bring together  larval abundance data for northern anchovy (Engraulis mordax) and Pacific sardine (Sardinops sagax) collected by United States and Mexican scientists over 50 years (1963-2015) to test the Basin and Asynchrony hypotheses. The Basin Hypothesis states that a species’ geographic range and spawning area (R) increases with overall abundance (A) according to a power law, R=aAb, where the exponent (b) is less than ~ 0.5 when the rate of increasing area occupied saturates as population size increases. The Asynchrony Hypothesis postulates that anchovy and sardine abundances are negatively correlated through time. We found that the Basin Hypothesis was supported for both species but the Asynchrony Hypothesis was not during this 53-year period.  Due to collaboration between U.S. and Mexican scientists we were able to better understand how two important fishes utilize their environment. 

Methods
CalCOFI and IMECOCAL collected fish larvae (ichthyoplankton) with obliquely-towed ring nets (1951 to 1977) and bongo nets (1978-present; Ohman and Smith, 1995; Thompson et al. 2017).  Detailed sampling protocols can be found in Kramer et al. (1972) and Smith and Richardson (1977), but, briefly, nets were towed to 140 meters (or within 15 m of the bottom at shallow stations) from 1951-1968 and then to 210 meters after 1968 (Ohman and Smith 1995).  Plankton in ring nets and starboard side of the bongo net were preserved in 5% buffered formalin, while, beginning in 1997, contents of the port side net were preserved in 100% ethanol (Gold et al. 2023); here, we focus on just formalin-preserved samples.  In the laboratory, all larval fishes were sorted from the rest of the plankton and identified to species.  Whereas it is not possible to identify all ichthyoplankton to species (e.g., rockfishes; Thompson et al. (2016)), anchovy and sardine are readily identifiable at all life stages. To account for any differences in tow depths or the amount of water filtered, raw abundances were standardized by multiplying by standard haul factor which accounts for the volume of water that was filtered (SHF; Smith and Richardson, 1977) and divided by the percent of the sample sorted (100% of most samples were sorted but in cases where zooplankton biomass was very high a fraction was sorted).  These adjustments make it possible to directly compare samples across the time series despite some changes to sampling methodology through time (Thompson et al. 2017). The units of abundance from the net tows are reported as “number under 10m2”, which is the number of larvae in a column of water with area 10m2 extending downwards to the depth of the tow.