Aleutian Islands - Ecological
Executive Summary
Full Report - Last updated 2018
- The Aleutian islands bottom trawl survey of structural epifauna showed variable distributions. Sponges are caught in most tows in the AI west of the southern Bering Sea. Abundance of coral in all areas has declined since about 1991-1993 surveys and is at generally low levels in all areas, but the frequency of occurrence has remained steady. Soft corals occur in relatively few tows, except in the eastern Aleutian Islands. Sea anemones are common, but sea pen abundance is low (p. 56).
- In the Bering Sea region north of the Western and Central Aleutian Islands that is sampled by the Continuous Plankton Recorder, spring diatom abundances have remained above average 2013–2017 (p. 60).
- However, copepod community size anomalies have remained negative from 2014–2017 indicating a community biased towards smaller species than is typical in this ecosystem (p. 60).
- Jellyfish mean catch per unit effort in the biennial AI bottom trawl survey is typically higher in the western and eastern AI than in the central AI. Catches and frequency of occurrence steadily increased across the AI from 2012 to 2016, but decreased in 2018 (p. 63).
- Length-weight residuals (a measure of groundfish condition) in 2014 and 2016 were negative for all species except arrowtooth flounder in 2014 and southern rock sole in 2016. In 2018, condition continued to be strongly negative for Pacific cod, northern rockfish, Pacific Ocean perch and arrowtooth flounder (p. 65)
- The depth distributions of rougheye rockfish, Pacific ocean perch, shortraker rockfish, and northern rockfish have been shallower in the most recent bottom trawl surveys of the Aleutian Islands. The mean spatial distribution of northern rockfish is trending westward. There are no significant trends in mean-weighted temperature distributions for any rockfish species (p. 67).
- Benthic communities and other non-target species sampled incidentally in the biennial bottom trawl survey. There has been a notable decline in eelpout biomass in the western AI in the last 4 surveys. Echinoderms and poachers have been increasing in the eastern AI ecoregion in the last 3 surveys (p. 71).
- The overall biomass in the summer bottom trawl survey in 2018 was very similar to that in 2016. The largest difference was a decrease in arrowtooth flounder, which occured in the central Aleutian Islands ecoregion (p. 8).
- Pacific cod showed a large decrease in the western Aleutian Islands ecoregion, but the overall pattern was balanced by increases in the eastern and central Aleutian Islands ecoregion (p. 8).
- The long term trend reflects a continuing shift in the pelagic foraging biomass from that dominated by Atka mackerel and pollock to that dominated by rockfish (p. 8)
- In general, seabirds in the Aleutians did not experience widespread failures like they did in the Gulf of Alaska during the marine heat wave of the past few years. However many piscivorous seabirds did poorly in 2018 at Buldir (western AI) and had mixed success at Aiktak (eastern AI), while planktivorous seabirds have remained generally successful. This pattern suggests that zooplankton availability was sufficient to support chick-rearing at both colonies, but that forage fish prey were insufficient to support chick-rearing at Buldir (p. 73).
- The western AI Steller sea lion adult population decreased rapidly at approximately 7% per year and sub-area population trends improved to the east through the western Gulf of Alaska, where the annual trend increased approximately 4% per year. Regional trends in pup production are similar to trends in non-pup counts, with continued relatively steep declines in the western AI, a less steep decline in the central AI, and improvement in the eastern AI (p. 2).
- A new indicator demonstrates that the stability (inverse biomass coefficient of variation) of groundfish biomass in the Aleutian Islands bottom trawl survey has been relatively constant from 2010–2018. There has been a gradual decrease over this time, but with a non-significant linear trend that is influenced by variability in Atka mackerel biomass (p. 76).
- A new indicator tracks fluctuations in the size of groundfish sampled over time by the Aleutian Islands bottom trawl survey. The mean length of the groundfish community in 2018 is the highest value over the time series, but in general has remained stable over time (p. 77).
- A new indicator tracks the mean life span of the groundfish sampled by the Aleutian Islands bottom trawl survey over time. This indicator serves as a proxy for the mean turnover rate of species and communities” and is intended to reflect ecosystem stability and resistance to perturbations. The mean lifespan in 2018 is above the long term mean, but has been largely stable over the time period with some interannual variation due to high biomass estimates of pollock or Atka mackerel (p. 79).