Monitoring the Milwaukee Harbor breakwater : an Engineering With Nature® (EWN®) demonstration project

The US Army Corps of Engineers (USACE) maintains breakwaters in Milwaukee Harbor. USACE’s Engineering With Nature® (EWN®) breakwater demonstration project created rocky aquatic habitat with cobbles (10–20 cm) covering boulders (6–8 metric tons) along a 152 m section. A prolific population of Hemimysis anomala, an introduced Pontocaspian mysid and important food source for local pelagic fishes, was significantly (p < .05) more abundant on cobbles versus boulders. Food-habits data of alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax) provided evidence that H. anomala were a common prey item. Night surveys and gill netting confirmed O. mordax preferred foraging on the cobbles (p < .05) and consumed more H. anomala than at the reference site (p < .05). H. anomala comprised a significant portion of the diets of young-of-the-year (YOY) yellow perch (Perca flavescens), YOY largemouth bass (Micropterus salmoides), and juvenile rock bass (Ambloplites rupestris) caught on the breakwater. The natural features’ construction on the breakwater increased the available habitat for this benthopelagic macroinvertebrate and created a novel ecosystem benefiting forage fish and a nursery habitat benefiting nearshore game fish juveniles. These data will encourage the application of EWN concepts during structural repairs at other built navigation infrastructure.

Coastal fishes in the northwestern Okhotsk Sea (westward from 147o E): fishery and prospects of its development

Fishery at the northwestern Okhotsk Sea coast is analyzed for the period from 2000 to 2019. Pacific herring, saffron cod, pacific cod, capelin, flounders (Limanda aspera, Pleuronectes quadrituberculatus and Platichthys stellatus), and smelts (Hypomesus japonicus and Osmerus mordax) are landed in this area. The spawning herring prevails (95.2 %) in the catch. Almost all spawning herring is caught at the coast of Khabarovsky Krai. Taking into account the area of spawning grounds for herring in the lagoons of the coasta, withdrawal of this species could be increased, though actually it was decreased in the last two decades in 1.8 times, and since 2012 the herring fishery is continued in Okhotsk District only. The fishing strategy changed: offshore harvesting with stationary nets transformed to combined using of stationary and casting nets and expanded to the rivers. The catches delivery changed, as well: the herring catches are processed mostly ashore since 2008. Withdrawal of saffron cod fluctuated from 1.9 % to 186.5 % of the total available catch (TAC) volume for this species, with rapid growth in the last 5 years; its main fishing grounds shifted from the waters of Nikolayevsk District to the waters of Tugur-Chumikan District. Landings of smelt grew since 2009, too: annual catches of rainbow smelt increased from 72.2 t, on average, in 2009–2014 to 119.4 t in the last 5 years (up to 90 % of this volume was caught in Tugur-Chumikan District); landing of japanese smelt grows even faster and now exceeds 50 % of TAC. The stock of capelin was exploited weakly, annual catches of this species changed from 0.7 % to 23.3 % of TAC. Stationary and casting nets were the only fishing gears used for the capelin fishery in 2000s, but recently midwater trawls are also used. Catches of flounders had a growth tendency, though they were rather low and never exceeded 50 % of the recommended volumes. Dynamics of the pacific cod landings became positive when fishermen started to use small boats for this fishery, its annual catches increased in 2014–2019 from 69.7 to 84.8 t, the withdrawal rate achieved 9.3 % of TAC. Available fish resources in the northwestern Okhotsk Sea and technical equipment of local enterprises provide strong potential of fishery industry in this area.

Rainbow smelt population responses to species invasions and change in environmental condition

Invasive species can have major disruptions on native food webs, yet the impact of species introductions and whether they will become invasive appears to be context-dependent. Rainbow smelt and alewife co-exist as invasive species in the Laurentian Great Lakes and as native species on the Atlantic coast of North America, but in Lake Champlain rainbow smelt is the dominant native forage fish and alewife are invasive. Alewife became abundant by 2007, providing an opportunity to explore the dynamics of these two species in a system where only one is invasive. We used data from a 31-year forage fish survey to compare demographics of rainbow smelt populations in three basins of Lake Champlain with different pelagic volumes, nutrient levels, and predator abundances. Rainbow smelt catch-per-unit-effort (CPUE) remained constant in the large, deep Main Lake before and after alewife invaded, but decreased in the two smaller basins. Declines were primarily a result of increased age-0 and age-1 mortality. Predation by top piscivores, system productivity, and resource competition alone could not explain the patterns in CPUE across the basins. The mechanisms that allow alewife and rainbow smelt to co-exist could be related to system volume and oxythermal habitat availability, and may explain why the two species do not negatively affect each other in other systems. Summer hypoxia in the smaller basins could force individuals into smaller habitat volumes with higher densities of competitors and cannibalistic adult smelt. Our findings suggest that habitat size mediates the impact of invasive alewife on native rainbow smelt.

Modern state of aquatic biological resources of the Amur River basin and directions of their research

Current status of aquatic biological resources in the middle and lower parts of the Amur River basin, including Lake Khanka and the Amursky Liman, is assessed. Generally high abundance of the water organisms is noted, but a downward trend is revealed. In 2015–2019, the total annual catch in the basin by Russian fishermen changed between 15.9–69.6 . 103 t (on average 39.1 . 103 t), with the main portions of pacific salmons (31.9 . 103 t, or 81.6 % of total catch) and smelts (5.5 . 103 t, 14.1 %). After the peak in 2016, the salmons abundance in the Amur has decreased, particularly for summer chum salmon and pink salmon, the number of arctic rainbow smelt Osmerus dentex has decreased gradually in the last 3 years. The stocks of freshwater fish are generally stable, with a slight increase for some species. The program of fisheries research «Amur River Fishes» implemented for 2020–2024 includes intensifying of traditional monitoring of the stocks and their biological state, as well as organization of detailed comprehensive studies for key species. Improvement of data quality on status of the main stocks of pacific salmons, smelts, and freshwater fishes is planned, as the basis for fisheries forecasting. Besides, the program conducts quantitative assessment of the main components of the ecosystem (phyto- and zooplankton, macrozoobenthos, and fish) and their dependence on environmental changes. State of food base for the Amur fish will be evaluated, including the feeding of artificially reproduced juveniles of salmons and sturgeons. Results of these studies will allow to improve approaches to regulation of fishery and to promote development of fishery industry toward organization of effective fishery complex in the Amur River basin.

Hypoxia’s impact on pelagic fish populations in Lake Erie: a tale of two planktivores

Whether bottom hypoxia has long-lasting consequences for pelagic fish populations remains speculative for most ecosystems. We explored hypoxia’s influence on two pelagic zooplanktivores in Lake Erie that have different thermal preferences: cold-water rainbow smelt (Osmerus mordax) and warm-water emerald shiners (Notropis atherinoides). To assess acute effects, we combined predictive bioenergetics-based modeling with field collections made across the hypoxic season in central Lake Erie during 2005 and 2007. To assess chronic effects, we related fishery-independent and fishery-dependent catches with hypoxia severity and top predator (walleye, Sander vitreus) abundance during 1986–2014. As our modeling predicted, hypoxia altered rainbow smelt movement and distributions, leading to avoidance of cold, hypoxic bottom waters. In response, diets shifted from benthic to pelagic organisms, and consumption and energetic condition declined. These changes were lacking in emerald shiners. Our long-term analyses showed rainbow smelt abundance and hypoxia to be negatively related and suggested that hypoxia avoidance increases susceptibility to commercial fishing and walleye predation. Collectively, our findings indicate that hypoxia can negatively affect pelagic fish populations over the long term, especially those requiring cold water.