| Abstract |
- A better understanding of the distribution and spawning behavior of Crangon shrimp and their relationship with their predators, including the green sturgeon, Acipenser medirostris, is an important step toward the larger goal to understand food webs of the nearshore continental shelf of the northeastern Pacific. Here, I examined how the biomass and energetics of two species of Crangon varied seasonally and by depth and the relationship between those patterns and the modeled distribution of A. medirostris, hypothesizing that the predicted presence of sturgeon would show a positive relationship with Crangon.
With the help of Larkin Loewenherz, a colleague, I also examined the assemblage and spawning behavior of Crangon species across several habitat types in Yaquina Bay, Oregon, hypothesizing that C. franciscorum would dominate the bay while C. alaskensis would be most common offshore, and that these species would be of larger average mass offshore due to mature individuals moving down the salinity gradient. This latter investigation, dealing with spawning and assemblage, was presented to a public audience as the final project for the spring term 2014 BI 450 Marine Biology course at Hatfield Marine Science Center.
Crangon samples were collected via beam trawl near Newport, Oregon, in 15 months over a span of 3 years (2010-2013). The presence of A. medirostris was predicted with help from Dr. David Huff, using a model based on data from coastal hydrophone arrays tracking acoustically tagged sturgeon, that included a number of physical covariates. For spawning and assemblage, samples were trawled from two stations in the river channel of Yaquina Bay and collected from two beach stations by seine net during spring 2014, and compared with offshore samples taken the previous year.
Subsequent analyses found that for the most part, biomass density of Crangon was higher in fall and late summer than in winter or spring, driven by the more abundant C. alaskensis. Alternatively, Crangon stylirostris was most abundant in February and at shallower depths. Energetics of individual C. alaskensis was higher in winter than in fall and at shallower depths, and energetics of C. stylirostris showed no effect from month or depth. Biomass density of Crangon and calorimetry of C. alaskensis were both tested as explanatory variables for A. medirostris, but no relationship was found between these quantities and A. medirostris presence.
For assemblage and spawning, we found that species assemblage differed between habitat types, but there was no significant difference in mean individual biomass between the channel and offshore habitats. We also found that there was no significant difference in average proportion of gravid C. alaskensis females between the channel and offshore habitats, leaving the question of where they go to spawn unanswered. We did find a positive correlation between the average total length and the percentage of gravid females.
The observed patterns in Crangon abundance and energetics could be explained either by seasonal fluctuation of nutrients caused by upwelling, or by reproductive migration of individual shrimp once they reached sexual maturity. The lack of any correlation between Crangon distribution and A. medirostris presence doesn’t rule out some relationship between the two, but a larger spatial area of Crangon sampling might be required to detect those patterns if they exist.
From our findings regarding assemblage and spawning, one of two possible interpretations exists: either salinity is not sufficiently different between the bay and the offshore area - or more likely, salinity is not a useful predictor of gravidity in Crangon.
This study was important because it aimed to shed some light on the habits of an oft-overlooked group of animals, which likely play a significant role in coastal food webs. By sharing our work with an audience that extended beyond our immediate colleagues and included people outside of our field, our hope was that we would open the eyes of some of the public to just how many questions about the denizens of the supposedly familiar near-shore and estuarine environments still wait to be answered.
- Keywords: Hatfield, Shrimp, green, Biology, Crangon, sturgeon, Population, Pacific, Estuarine, medirostris, Yaquina, Gravidity, Spawning, calorimetry, Estuary, Ecology, Caridea, Gravid, acipenser, Benthic, Marine
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