SEASONAL AND INSHORE-OFFSHORE VARIATIONS IN THE 



STANDING STOCKS OF MICRONEKTON AND 



MACROZOOPLANKTON OFF OREGON 



William G. Pearcy^ 



ABSTRACT 



Dry weights of pelagic animals captured along an inshore-offshore station line with Isaacs-Kidd 

 mid-water trawls and 1-m diameter plankton nets during a 5-yr period provided evidence for seasonal 

 changes in the standing stocks of carnivores. Micronekton catches (fishes, shrimps, and squids) were 

 largest inshore (28 and 46 km offshore) in the winter (November-April), and offshore (84 and 120 km) 

 during the summer (May-October), the season of coastal upwelling. No seasonal difference was 

 detected in the biomass of herbivores, or in its primary components, the copepods and euphausiids. 

 Increased biomass of medusae during the summer resulted in significant seasonal differences in the 

 planktonic carnivores at the inshore stations. 



The average biomass (grams per square meter) of small nektonic and planktonic carnivores, 

 averaged over the year, peaked at the 84-km station. The biomass of fishes was greater than shrimps 

 and the biomass of shrimps was greater than that of squids at all stations, except 46 km where shrimps 

 predominated. Herbivore biomass was maximal at 46 km, over the inner continental slope, largely 

 because of the high catches of euphausiids at this station. The occurrence of largest average catches at 

 intermediate distances from shore, and inshore-offshore shifts in peak biomass with seasons, may 

 result from seasonal changes in upwelling and downwelling and exclusion of vertical migrants 

 from shoal waters on the shelf 



Herbivore: carnivore biomass ratios differed significantly between inshore and offshore stations. 

 Standing stocks of herbivores were several times larger than those of carnivores in nearshore waters, 

 but the ratio was about 1.0 in offshore waters. Coefficients of variation (s/x) of herbivore and plank- 

 tonic carnivore stocks for the entire sampling period were highest inshore, indicating high variabil- 

 ity, and decreased markedly in offshore waters. These trends suggest that, compared to offshore or 

 oceanic communities, the pelagic inshore-upwelling ecosystem may be less predictable and have a 

 lower ecological efficiency. 



This research was designed to answer two ecologi- 

 cal questions about intermediate consumers in the 

 pelagic food chain off Oregon: 



(1) Are seasonal variations obvious in the 

 standing stocks of small nekton and macrozoo- 

 plankton off Oregon, perhaps in response to up- 

 welling along the coast during the summer? 



(2) Are there trends in the standing stocks of 

 these animals from oceanic waters into neritic 

 waters and, if so, do they reflect basic ecological 

 differences in these pelagic communities? 



Pelagic animals such as fishes, squids, shrimps, 

 and euphausiids are ubiquitous in the open oceans 

 and are important intermediates in the food chain 

 between small plankton and large pelagic carni- 

 vores. Yet little is known about their seasonal 

 variations, inshore-offshore distributions, or gen- 



'School of Oceanography, Oregon State University, Corval- 

 lis, OR 97331 



eral ecology. The life span and generation time of 

 many of these intermediate consumers are 1 

 yr or greater, limiting short-term changes in 

 population sizes. Moreover, many of these ani- 

 mals reside below the depth of seasonal tempera- 

 ture change much of the time. They may under- 

 take diel vertical migrations, and some species 

 may migrate through the thermocline at night. In 

 any event, seasonal changes in physical environ- 

 ment are expected to be less pronounced than 

 those experienced by inhabitants of surface 

 waters. Thus, seasonal variations in population 

 size of these animals are expected to be less 

 than those of small planktonic organisms. 



Movements of water may also affect seasonal 

 changes in the abundance of animals at one 

 locality, or spatial distributions within a general 

 region. In areas where water masses and as- 

 sociated pelagic fauna overlap and mix, species 

 structure may be complicated, primarily a result 



Manuscript accepted April 1975. 



FISHERY BULLETIN: VOL. 74, NO. 1, 1976. 



70 y-^ - ^^ 



