Experimental Examinations of Temperature Interactions in the ‘Match-Mismatch’ Hypotheses Using Pacific Cod Larvae
Figure 3. Adult Pacific cod caught on jigging gear aboard the fishing vessel Miss O in April 2007. Photo by Benjamin Laurel.
Figure 4. Fertilized batches of Pacific cod being prepared for shipment back to the FBEP lab in Newport, Oregon. Photo by Benjamin Laurel.
Figure 5. Changes in larval mass at two temperatures under varying ‘match’ conditions (i.e., high food (HF)) and varying mismatch conditions (i.e., low food (LF), (3) HF then LF (HF-LF), and (4) LF then HF (LF-HF)). The dotted line indicates the timing of prey switch in the HFLF and LFHF treatments. Most growth variance is explained by temperature (note differences in scale along the y-axis between top and bottom panel).
Figure 6. Pacific cod larvae (3 weeks post-hatch) reared under high food conditions at 8°C. Photo by Benjamin Laurel.
The temporal synchrony of marine fish larvae and zooplankton is being severely altered as the result of changing climatic conditions. Pacific cod have an extremely narrow spawning window in the spring (single-batch spawners) and may, therefore, be particularly susceptible in the timing of prey production and changing temperature field.
At the AFSC’s Fisheries Behavioral Ecology Program (FBEP) laboratory at the Hatfield Marine Science Center in Newport, Oregon, experiments have been under way to examine the response of Pacific cod larvae to ‘matches’ and ‘mismatches’ in prey availability in cold and warm years.
To collect eggs, we have repeatedly chartered cod-jigging vessels out of Kodiak, Alaska, to catch Pacific cod off their spawning grounds in early spring (Fig. 3). Female and male gametes were stripped and eggs were fertilized at sea. Fertilized egg batches (Fig. 4) were then shipped back to the FBEP laboratory to begin experimental feeding trials.
In the laboratory, we manipulated timing and magnitude of prey introduction under cold (3°C) and warm (8°C) conditions over a 6-week period. Larvae were reared in 100-L tanks on enriched rotifers, but at week 3, half of the feeding treatments were switched from either a low-food (LF) to a high-food (HF) prey density or from a HF to a LF prey density. Remaining treatments were maintained on a LF or HF ration throughout the 6-week period.
Early into the experiment (week 3), it was evident that temperature, not timing or prey abundance, explained the majority of growth variation in this species (Figs. 5 and 6). However, at high temperatures, significant effects of prey abundance were detectable at the end of the experiment (6 weeks; Fig. 5).
Prey timing (match-mismatch) was demonstrated to be important only if 1) it occurred at high temperatures and 2) mismatches in prey occurred after the 3-week prey switch, largely due to the buffering effects of the yolk-sac period and compensatory growth mechanisms of the larvae.
The effects on survival were also interesting. Despite some variance explained by prey level and prey timing, most of the survival variance was due to temperature (i.e., warm temperatures resulted in a 2-4× survival increase (Fig. 7)). Therefore, even in the absence of predation, there appears to be a strong effect of temperature on stage-duration mortality.
These experiments support the idea that strong year classes of Pacific cod will be positively linked with temperature in the North Pacific. However, there are no guarantees. While Pacific cod can increase their growth rates following early prey mismatches, larval mortality in the wake of a complete absence of prey would be magnified under warm conditions.
For example, based on yolk-reserves alone, earlier work in our lab indicated Pacific cod larvae would not be able to take advantage of the late blooms in the Bering Sea if peak larval supply precedes the bloom by more than 2 weeks, whereas at colder temperatures (e.g., 0°C), this window of opportunity is nearly doubled.
Collectively, our results demonstrate a clear need to consider the physical environment, both as a driver of lower level productivity and as a major factor influencing the physiology and food requirements of larvae interacting with their prey.
