Burkett 



Chapter 22 



Food Habits and Prey Ecology 



and phosphorescent and closely resemble shrimps in form 

 though they are often not more than 25 mm long. The 

 phosphorescent organs are along the sides of the body. 

 Their purpose is not known. Attached to the thorax are the 

 eight pairs of two-branched legs which give rise to the name 

 "schizopoda," as this order was formerly called (Johnson 

 and Snook 1967: 293-294). 



Zooplankton are found in greater abundance during cold- 

 water years in California waters. Many of the zooplankton 

 are predators on fish eggs and larval fish, and their abundance 

 was sometimes twenty times greater during the colder periods 

 (Reid and others 1958). Accordingly, not only would the 

 lowered temperature affect survival of fish eggs and larvae 

 directly, it would also add to the hazards of being eaten by 

 providing conditions for the rapid increase of zooplankton 

 (Ricketts and Calvin 1962: 394). 



Komaki (1967) summarized information obtained from 

 fishermen on the phenomenon of surface swarming of 

 euphausiids (E. pacifica) in the Sea of Japan. This phenomenon 

 differs from the usual vertical migratory behavior because it 

 occurs in the daytime, independent of light intensity. The 

 swarming season in the Kinkazan waters ranged between 

 late February and late May. Water temperature was determined 

 to be the most important factor, with swarming starting at a 

 slightly higher temperature than the local minimum (7 degrees 

 Celsius), continuing with increasing temperature, and then 

 terminating as the temperature exceeded 16 degrees Celsius. 

 Because swarming did not occur earlier in the year when 

 temperatures were favorable, Komaki (1967) concluded that 

 the swarming was related to reproduction. Also, it appeared 

 that the population was composed of several stocks, and that 

 as stocks reached a certain degree of maturity, they approached 

 the coast in succession. 



The daily phenomenon of vertical migration was noted 

 as early as 1872 during the Challenger Expedition. Many 

 plankton animals actively move towards the surface of the 

 ocean at night and sink or swim away to the depths in the 

 daytime. Vertical climbing requires much energy and has 

 been developed so frequently in the animal kingdom that it 

 was thought to clearly be of some significance in the lives of 

 such animals (Hardy 1965: 199-200). The main proximate 

 factor for daily vertical migration appears to be light intensity 

 (Gushing in Raymont 1963: 435). 



Both E. pacifica and T. spinifera were found to undergo 

 vertical migration off Washington State in summer 1967 

 (Alton and Blackburn 1972). High catch rates were sustained 

 from near-surface water throughout the late evening and 

 early morning hours, approximately 2200 to 0500 hours. 



Hardy (1965: 212-215) advanced a general theory for 

 the value of diurnal vertical migration. Because the uppermost 

 layers of the sea generally move at higher speeds than lower 

 levels and bottom topography results in currents which may 

 differ from surface layers, the regular movement of plankton 

 between these layers allows the animals to be carried over 

 greater distances than would otherwise be the case. Thus, the 

 plankton population can be distributed over a much larger 



area of the ocean than if continually moved by only one 

 body of water. This large-scale movement has the advantage 

 of putting the animals in contact with more food source 

 patches. Individual variation in the degree of vertical migration 

 and the amount of time spent at any one layer further promote 

 the patchy distribution of plankton. 



A genetic theory has also been proposed (David in 

 Raymont 1963: 466). Marine planktonic species may tend to 

 become divided into relatively small, separate populations if 

 continually drifting in one stratum and not normally 

 encountering directional stimuli to encourage horizontal 

 migration. However, the broader distribution caused by 

 vertical migration would help to encourage interchange of 

 zooplankton populations and thus promote gene flow. 



There are many other theories regarding vertical 

 migration. The fact that both Raymont (1963) and Hardy 

 (1965) devoted an entire chapter to the subject attests to the 

 complexity of factors which operate in the marine 

 environment. As summarized by Hardy (1965: 217): "There 

 can be no doubt that the patchy distribution of the plankton 

 must be due to a great variety of causes." Raymont (1963: 

 466) ended his chapter by recognizing the need for more 

 research on the subject: "At this stage no conclusive answer 

 can be given to the question as to the value of diurnal 

 vertical migration, but the tremendously wide occurrence of 

 this phenomenon in the seas is one of the most challenging 

 aspects of marine plankton study." 



With the variability in zooplankton distribution and 

 abundance, the way in which murrelets find such prey resources 

 warrants attention. The interannual variability in euphausiid 

 consumption (tables 2 and 3) noted by Krasnow and Sanger 

 (1982) could demonstrate differences in zooplankton 

 distribution (rather than abundance) and the corresponding 

 inability of murrelets to locate and use the resource. However, 

 some of the distribution patterns should be predictable at 

 least between "normal" years, and thus the learning of foraging 

 areas by murrelets would indeed be important for minimizing 

 energy expenditure as suggested by Carter (1984). Komaki 

 (1967) demonstrated that E. pacifica and sand eels (A. 

 personatus) fluctuated in parallel on the basis of data on 

 fishery harvest, and that sand eels were taken in almost the 

 same area as the euphausiid fishery. The traditional euphausiid 

 swarming areas were known to the fishermen, though the 

 density of the swarm varied between years (Komaki 1967). 

 Thus, conservation of the murrelet and its food web will be 

 aided by identification and appropriate management of 

 important euphausiid swarming areas, especially in the vicinity 

 of known murrelet nesting areas. 



Pacific Sand Lance 



Sand lance are slim, elongated, usually silver fishes 

 especially abundant in northern seas. They belong to the 

 ammodytidae and are sometimes called sand eels, but they 

 are not true eels even though eel-like in shape and 

 movement. The Pacific sand lance is distributed from 

 southern California to Alaska and to the Sea of Japan. 



234 



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



