Estimating forage fish distribution and abundance using aerial surveys in Prince William Sound, Alaska: 1995 - 1999

[Missing plankton, jellyfish, seine, trawl, bird and CTD data sets from this study]

Sound Ecosystem Assessment (SEA) aerial surveys, 1995 - 1997:
We show that aerial visual surveys are a cost-effective method to assess distribution and abundance of forage fish and seabirds and produce repeatable results with an acceptable level of error. We fulfilled our main objectives concerning forage fish assessment: 1) develop an appropriate survey design incorporating detection error, surveyor bias, and survey constraints, 2) determine the accuracy of fish species and age class discrimination, 3) determine the repeatability of aerial measurements, and 4) determine the spatial resolution of aerial measurements needed for ecological analyses. We also fulfilled our secondary objective to develop abundance estimates for jellyfish aggregations and seabirds. We developed an adaptive modified line transect methodology and model incorporating a detection curve for Pacific herring (Clupea pallusi), Pacific sand lance (Ammodytes hempems), and black-legged kittiwakes (Rissu triductylu) and a detection correction factor for fish schools. We established survey condition constraints and an optimal survey altitude range of 274 - 366 m based on school size and sightability of seabirds. We developed logging software to obtain flight path and sighting coordinates and incorporated measurements of schools using a sighting tube. The altitude dependent flight swath or transect width for fish schools was estimated from the detection curve at 474 m at an altitude of 305 m. The detection correction factor was estimated at 0.727 resulting in an undercounting rate of approximately 37.5%. Using double counting, individual surveyor bias was estimated at 6.5% for school counts and 7.0% for school surface area estimates; these rates are lower than those reported for wildlife aerial surveys. Error in fish species identification was estimated from validation data at 3.9% for herring, and 20. 2% for sand lance; we were not able to discriminate age classes of herring and a model did not improve discrimination. We recommended methods to reduce the cost of obtaining validation samples for future surveys. We determined the precision of fish abundance estimates by performing repeat flyovers; our error rate was 14.6% for school counts and 18.3% for total school surface area estimates within a 36 hr period; surveyor bias is imbedded in this estimate. The variance of the density estimates will be mainly influenced by the variability of biological occurrence since the variance of the detection correction factor including surveyor bias was very low. The additional contribution of error from identification and repeatability of measurements is species specific. For herring the additional error for schools counts, 12 - 18.5%, and surface area estimates, 15.2 - 22.2%, should be applied depending on whether or not the redundancy for surveyor bias is removed. For sand lance, the additional error is 27.7 - 34.2% for school counts and 30.9 - 37.9% for school surface area estimates. The spatial resolution of our data was established at approximately 500 m, but could be reduced with uninterrupted coordinate logging to 80 m. If a finer resolution or degree of precision is required, the technique can be combined with electronic imaging methods reviewed here. Finally, we recommended a field survey design combining new and old technology to provide cost-effective, yet reasonably accurate and precise results.

Alaska Predator Ecosystem Experiment (APEX) surveys, 1998 - 1999:
Study History: Restoration Project 99163-T was initiated after a round of data sharing between the Sound Ecosystem Assessment (SEA) project, Restoration Project 99320, and the Alaska Predator Ecosystem Experiment (APEX) project, Restoration Project 99163. In the 1998 annual meeting and review for Exxon Valdez Oil Spill (EVOS) science the principal investigator of this project present data on broad-scale distributions of surface schooling forage fishes in Prince William Sound from 1995-1997. In addition, modelers working with the APEX project found a significant correlation between foraging activity of sea birds and the fish distributions from the aerial surveys. It was decided at that meeting that an aerial component should be added to the APEX program to provide additional information on fish distribution for modelers and other researchers with the APEX umbrella. Therefore, this project was conceived to as a service and data delivery project for that program. The first field season was 1998 and a second (final) field season was completed in 1999.

Abstract: The objective of this project was to provide information on pelagic schooling fishes in the surface waters of Prince William Sound in order to better understand reproductive and foraging dynamics of various sea bird species. The scope of the project focuses on the study areas of the APEX project within Prince William Sound, Alaska. The individual objectives for this project were completed between the dates of July 1 and August 9, 1998. Preliminary data was delivered to the APEX program via Glen Ford, APEX modeler. Net catches from our survey and other APEX projects were compiled to provide corrections for species identification. Final edited data was delivered on December 14, 1998. We were not able to over fly the acoustics program within APEX due to a non- overlap in over flights and cruises. However, we were able to conduct a single broad-scale survey within Prince William Sound during the course of July, 1998. In meeting our objectives, we flew 15 repeat surveys over the northern and central study regions and 5 repeat surveys (coordinating with kittiwake researchers) over the Jackpot study region (coordinating with marbled murrelet researchers).

Publications:
Ainley, D. G., Ford, R. G., Brown, E. D., Suryan, R. M., & Irons, D. B. (2003). Prey resources, competition, and geographic structure of kittiwake colonies in Prince William Sound. Ecology, 84(3), 709-723.

Brenda L. Norcross, Evelyn D. Brown, Robert J. Foy, Michele Frandsen, Jody Seitz and Kevin Stokesbury. 1999. Juvenile Herring Growth and Habitats, Exxon Valdez Oil Spill Restoration Project Final Report (Restoration Project 99320T), Institute of Marine Science, University of Alaska-Fairbanks, Fairbanks, Alaska.

Brown, E.D. and B.L. Norcross. 1999. Aerial Survey Support for the APEX Project, Exxon Valdez Oil Spill Restoration Project Annual Report (Restoration Project 97163T), Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, Alaska

Irons, D.B. and R.M. Suryan. 1996. Kittiwakes as indicators of change in forage fish. Preliminary results of 1995 field season. APEX Component 95163E, US Fish andWildlife Service, 1011 E. Tudor Rd. Anchorage, AK 99503.

Purcell, J.E., E.D. Brown, K.D.E. Stokesbury, L.H. Haldorson, and T.C. Shirley. 2000. Aggregations of the jellyfish Aurelia labiata: Abundance, distribution, association with age-0 walleye pollock, and heavers promoting aggregation in Prince William Sound, Alaska, USA. Marine Ecology Progress Series. 195:145-158.