Burkett 



Chapter 22 



Food Habits and Prey Ecology 



long time scales of several centuries or more, but the 

 correlation of shorter-period components in the time series 

 is virtually nil. 



Baumgartner and co-authors say that caution in 

 interpreting the data should be exercised on two fronts: (1) 

 sample size; they acknowledge that additional samples are 

 needed to capture the complete range of variability of the 

 SDR's over the basin. (2) The collapse and recovery 

 demonstrated for the sardine do not necessarily mean that 

 the current cycle of collapse and recovery has no relation to 

 the application/release of fishing pressure or change in ocean 

 climate, or both. They infer that even though the causes may 

 vary (biological interaction, environmental change) for 

 different recoveries or collapses, the sustained reproductive 

 consequences are similar from one event to another 

 (Baumgartner and others 1992). 



Analysis of fish scales in sediments of the central Gulf 

 of California resulted in similarities with the Santa Barbara 

 Basin work (Holmgren-Urba and Baumgartner 1993). The 

 reconstructions show a strong negative association between 

 the presence of sardines and anchovies, with anchovies 

 dominating throughout the 19th century, and with only two 

 important peaks of sardine scale deposition. The two episodes 

 of sardine scale deposition occur virtually 1 80 degrees out 

 of phase with anchovy scale deposition. This suggests an 

 overall coherent pattern in changing ecosystem structure 

 that operates over a period of about 120 to 140 years. The 

 collapse of the sardine population in the Gulf of California 

 was very similar to the collapse in the California Current 

 during the late 1940's and 1950's. Both populations declined 

 under heavy fishing pressure (Barnes and others 1992) 

 superimposed on broad, natural, decadal-to-centennial-scale 

 biomass fluctuations (Soutar and Isaacs in Holmgren-Urba 

 and Baumgartner 1993). Both declines appear to be 

 accompanied by an increasing population of northern anchovy 

 (MacCall and Praeger in Holmgren-Urba and Baumgartner 

 1993). The relationship to climate was not entirely clear, but 

 suggested a mediating effect on population sizes. However, 

 the process is still subject to strong filtering through biological 

 interaction among species. 



Butler and others (1993) modeled anchovy and sardine 

 populations to examine how natural variation of life-history 

 parameters affected per capita growth. The greatest change 

 in growth for both species occurred during larval stages. A 

 number of important life history parameters of marine fish 

 are directly affected by changes in temperature, and 

 temperature and food densities affect growth at all stages. 

 For anchovies, there is some evidence that reproduction is 

 drastically reduced during major El Nino events. Under such 

 conditions, the anchovy stock declined. For the sardine, high 

 fishing mortality reduces the abundance of the oldest age 

 classes, which have the highest reproductive potential because 

 of their larger size and greater number of spawnings. Density- 

 dependent factors such as cannibalism on eggs may also be 

 important (Valdes and others and Valdes Szeinfeld in Butler 

 and others 1993). The results of this modeling exercise 



parallel the results and conclusions of McGurk and Warburton 

 (1992) described earlier in the section under sand lance. 



Structural changes over time in the California Current 

 ecosystem between sardines and anchovies are similar to 

 changes between herring and sand lance described previously 

 for the North Sea and the Atlantic, though different factors 

 were probably operative. Additionally, most researchers have 

 found it difficult to separate the effects of humans from 

 natural influences on the fish stocks. The fact that both 

 mechanisms will continue to operate dictates that managers 

 conduct effective monitoring programs and adaptive 

 management to allow prompt remedial action to be taken 

 where necessary (Wilson and others 1991). 



The low occurrence of sardines in the diet of murrelets 

 is interesting given the wide geographic distribution of this 

 fish (table 1). This low occurrence may be due to fewer 

 studies in the southern end of the murrelet's geographic 

 range where sardines are more abundant. Alternatively, it 

 may represent an overall lower abundance due to overfishing, 

 competition, and natural influences. Anderson and Anderson 

 in Anderson and others (1980) suggested that past breeding 

 populations of Brown Pelicans in the Southern California 

 Bight probably had a larger prey base than the existing 

 anchovy-dominated diet, perhaps also importantly involving 

 Pacific sardines and Pacific mackerel. Recent increased 

 abundance of sardines off southern California was followed 

 by increased breeding success and abundance of Brown 

 Pelicans (Ainley and Hunt in Anonymous 1993). 



Because of the natural fluctuations in anchovies and 

 sardines as shown from the scale-deposition studies, murrelets 

 probably evolved to use this resource in proportion to 

 availability. Thus, the periodic lows in anchovy and sardine 

 populations would probably not adversely affect the murrelet 

 as long as alternative forage fish remained available. 

 Development of new fisheries (sand lance or euphausiids) 

 and escalation of harvests for rockfish and herring would be 

 expected to affect murrelets, especially in conjunction with 

 a low period of anchovies and sardines, and El Nino events. 



Pacific Herring 



Herring belong to the clupeidae as do the Pacific sardine. 

 Adults range up to 45 cm in length (Miller and Lea 1972: 

 54). Herring are one of the most abundant species of fishes 

 in the world and prey upon copepods, pteropods, and other 

 planktonic crustaceans, as well as fish larvae. They travel in 

 vast schools, providing food for larger predators. 



The Pacific herring ranges from Baja California to Alaska 

 and across the north Pacific to Japan. Within this range, 

 abundance generally increases with latitude and the largest 

 populations are centered off Canada and Alaska (Spratt 1 98 1 ). 



Currently, all herring commercially harvested in California 

 and Oregon are taken as sac-row for Japanese markets. In 

 British Columbia and Alaska, herring are primarily harvested 

 for sac-row, and as longline bait (McAllister, pers. comm.). 



Spawning begins during November in California and 

 ends during June in Alaska, becoming progressively later 



238 



USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



