Biology and Ecological Niches in the Gulf of the Farallones 



Krill 



Dan Howard 



Just as there are growing seasons on land, 

 so there are growing seasons in the ocean as 

 well. In the Gulf of the Farallones, the grow- 

 ing season begins in early spring, when the 

 first phytoplankton blooms (large increase of 

 microscopic plants) of the year fuel growth 

 at higher levels of the marine food web. In 

 California, as in many parts of the world, 

 euphausiid shrimp, commonly called "krill," 

 are one of the beneficiaries of this early-sea- 

 son production and are a critical link in the 

 marine food web. Feeding on phytoplankton 

 (microscopic plants) and small zooplankton 

 (animals), krill populations expand and by 

 being eaten by other marine animals, trans- 

 fer energy from the lowest (primary pro- 

 ducer) level into the upper levels of the 

 marine food web. They are often referred 

 to as "keystone" species because they play 

 such an important role in the functioning of 

 many marine ecosystems. 



Krill hatch from free-floating eggs and 

 pass through larval and juvenile stages before 

 maturing into adults. This development pro- 

 cess involves a series of molts (casting off the 

 rigid outside skeleton that restricts growth), 

 during which segments and appendages are 

 gradually added. While the new outside skel- 

 eton is still soft, the individual can increase 

 its size. Adult euphausiids have the unique 

 ability to actually shrink in size after a molt 



if food resources are scarce. Because krill 

 can increase and decrease their size, it can 

 be difficult to determine their age or the age 

 distribution of a population of animals from 

 their sizes. 



Krill have legs called "swimmerets" that 

 have evolved to look like small feathers and 

 function like fins, giving them great mobility 

 and agility for life in the water column. They 

 feed while swimming, using their modified 

 front legs to form a food basket that strains 

 food from the water while they swim. 



Krill are typically very gregarious, which 

 means they are often found in large, concen- 

 trated groups, including dense swarms with 

 as many as 100,000 krill per cubic yard 

 of ocean water. This swarming behavior 

 makes krill especially vulnerable to preda- 

 tors. Swarming activity starts sometime in 

 spring and continues through the fall. These 

 aggregations can commonly be located in the 

 Gulf of the Farallones by finding flocks of 

 diving seabirds or clusters of birds picking 

 at the surface. Fishermen use these feeding 

 flocks to help them locate salmon feeding on 

 the undersides of the krill swarms. Lunging 

 humpback whales that break the surface of 

 the water are also a good indication that 

 krill are swarming at or near the surface. 

 Surface swarms provide ideal feeding condi- 

 tions for these large filter feeders. With its 



huge mouth, a gaping humpback (and other 

 baleen whales) is capable of engulfing a large 

 volume of krill in a single gulp. These dense 

 aggregations of prey provide the several tons 

 of food per day required by these whales 

 during the summer feeding season. 



In the Gulf of the Farallones, the two most 

 abundant species of krill are Thysanoessa spi- 

 nifera and Euphausia pacifica. They are typi- 

 cally about 3/4 inch long and live for about 2 

 years. Thysanoessa spinifera is found mostly 

 in shallower water over the continental shelf, 

 whereas Euphausia pacifica is usually found 

 in deeper water towards the margin of the 

 shelf and beyond. Between them they are a 

 major food source for salmon (krill pigments 

 give salmon flesh its characteristic pink to 

 orange color), rockfish, seabirds, and whales. 

 Krill are the main reason hungry humpback 

 and blue whales visit the gulf in the sum- 

 mer to fatten up for the rigors of the 

 coming year. Both of these krill species dem- 

 onstrate special adaptations that enable them 

 to succeed despite constant predation pres- 

 sure. From spring through fall, Thysanoessa 

 spinifera swarm at the surface during the 

 day. Though it is unclear what is driving this 

 behavior, the benefits must outweigh the cost, 

 which is increased predation. 



Euphausia pacifica, controlled by light 

 intensity, migrate out of ocean depths and 



into surface waters each night. As dawn 

 approaches, they return to deep water for 

 the day. This migration pattern is shared 

 with many other organisms in the ocean, and 

 during the day krill form part of the so-called 

 "deep scattering layer" that fishermen see 

 on their depth sounders. The timing of this 

 daily event in response to changing light 

 intensities provides Euphausia pacifica with 

 several advantages. By moving upward at 

 night, Euphausia pacifica minimizes expo- 

 sure to daytime predators, while grazing in 

 surface waters where food is abundant. They 

 also realize an energy gain by returning 

 during the day to the colder deep water, 

 where metabolism slows. Releasing eggs in 

 wanner surface waters speeds development 

 times, thus reducing the time exposed to 

 predators; it also ensures that hatching larvae 

 are in productive waters when they start feed- 

 ing, increasing their chances of survival. 



Krill are a critical link in the Gulf of the 

 Farallones marine food web and in marine 

 food webs around the world. They directly or 

 indirectly support the survival and well-being 

 of many animals living in different oceans. 

 Knowing the key position filled by krill in 

 many marine ecosystems, we need to ensure 

 that their populations remain healthy for the 

 well-being of all. 



36 Biology and Ecological Niches in the Gulf of the Farallones 



