The role of carrion supply in the abundance of deep-water fish off California

Abstract

Few time series of deep-sea systems exist from which the factors affecting abyssal fish populations can be evaluated. Previous analysis showed an increase in grenadier abundance, in the eastern North Pacific, which lagged epibenthic megafaunal abundance, mostly echinoderms, by 9-20 months. Subsequent diet studies suggested that carrion is the grenadier's most important food. Our goal was to evaluate if changes in carrion supply might drive the temporal changes in grenadier abundance. We analyzed a unique 17 year time series of abyssal grenadier abundance and size, collected at Station M (4100 m, 220 km offshore of Pt. Conception, California), and reaffirmed the increase in abundance and also showed an increase in mean size resulting in a ∼6 fold change in grenadier biomass. We compared this data with abundance estimates for surface living nekton (pacific hake and jack mackerel) eaten by the grenadiers as carrion. A significant positive correlation between Pacific hake (but not jack mackerel) and grenadiers was found. Hake seasonally migrate to the waters offshore of California to spawn. They are the most abundant nekton species in the region and the target of the largest commercial fishery off the west coast. The correlation to grenadier abundance was strongest when using hake abundance metrics from the area within 100 nmi of Station M. No significant correlation between grenadier abundance and hake biomass for the entire California current region was found. Given the results and grenadier longevity, migration is likely responsible for the results and the location of hake spawning probably is more important than the size of the spawning stock in understanding the dynamics of abyssal grenadier populations. Our results suggest that some abyssal fishes' population dynamics are controlled by the flux of large particles of carrion. Climate and fishing pressures affecting epipelagic fish stocks could readily modulate deep-sea fish dynamics.

Figure 1. Stations used for analysis from the California Cooperative Fisheries Investigation (CalCOFI) fish egg surveys.

The California coastline is depicted from San Diego in the south to San Francisco to the north. Several of the sampling line numbers are shown for reference. Station M is shown by the star.

Figure 2. Size frequency distributions of Coryphaenoides spp. showing an increase in mean size from 1990–92 to 2004–2005.

Figure 3. Time series of grenadier and epipelagic nekton egg abundances.

Grenadier abundance (thick black line) and (a) hake and (b) jack mackerel egg abundance for the larger sampled area (gray dots), the annually sampled region (thin gray line) and only stations within 100 nmi of Station M (thick gray lines) are shown.

Figure 4. Correlation coefficients (black) and associate p values (grey) between grenadier and nekton egg abundances.

Correlations were lagged from +12 (eggs leading grenadiers) and −12 (grenadiers leading eggs) months. Correlations to hake (A) and jack mackerel (B) from the 100 nmi (diamonds), regularly sampled area (circles) and larger area (squares) are shown. Solid black symbols for correlation coefficients indicate a significant correlation (p<0.05 as shown by the corresponding grey symbols).

Figure 5. Time series of yearly spawning stock biomass of hake and yearly mean grenadier abundance at Sta. M.