Years Of Low Abundance Research Articles

Synchrony erodes spatial portfolios of an anadromous fish and alters availability for resource users.

Environmental forces can create spatially synchronous dynamics among nearby populations. However, increased climate variability, driven by anthropogenic climate change, will likely enhance synchrony among spatially disparate populations. Population synchrony may lead to greater fluctuations in abundance, but the consequences of population synchrony across multiple scales of biological organization, including impacts to putative competitors, dependent predators or human communities, are rarely considered in this context. Chinook salmon Oncorhynchus tshawytscha stocks distribute across the Northeast Pacific, creating spatially variable portfolios that support large ocean fisheries and marine mammal predators, such as killer whales Orcinus orca. We rely on a multi-population model that simulates Chinook salmon ocean distribution and abundance to understand spatial portfolios, or variability in abundance within and among ocean distribution regions, of Chinook salmon stocks across 17 ocean regions from Southeast Alaska to California. We found the expected positive correlation between the number of stocks in an ocean region and spatial portfolio strength; however, increased demographic synchrony eroded Chinook salmon spatial portfolios in the ocean. Moreover, we observed decreased resource availability within ocean fishery management jurisdictions but not within killer whale summer habitat. We found a strong portfolio effect across both Southern Resident and Northern Resident killer whale habitats that was relatively unaffected by increased demographic synchrony, likely a result of the large spatial area included in these habitats. However, within the areas of smaller fishing management jurisdictions we found a weakening of Chinook salmon portfolios and increased but inconsistent likelihood of low abundance years as demographic synchrony increased. We suggest that management and conservation actions that reduce spatial synchrony can enhance short-term ecosystem resilience by promoting the stabilizing effect multiple stocks have on aggregate Chinook salmon populations and overall resource availability.

Dynamics of Linear Growth Rate in Herring, Clupea pallasii, from Peter the Great Bay, Sea of Japan

An analysis of the size and age composition of the Pacific herring Clupea pallasii from Peter the Great Bay in 1941, 1944, and 1947−2017 has shown the categories of large-, medium-, and small-sized fish that statistically significantly differ in linear size. Their proportions in year-classes are subject to long-term and interannual variations. The identified groups of fish are distinguished by life cycle duration, timing of sexual maturity, and abundance dynamics, but they do not have a complete reproductive isolation. In the long-term dynamics of proportions of the size categories, large- and medium-sized individuals with positive linear growth deviations dominate the considered year-classes during the periods of stock increase; groups of small- and medium-sized fish with negative deviations of growth indices prevail in the years of low abundance.

Resource partitioning confirmed by isotopic signatures allows small mammals to share seasonally flooded meadows.

Meadows in river deltas are characterized by a high diversity and abundance of small mammals. However, neither their spatial arrangement nor differences in their use of microhabitat can necessarily explain the dense co‐occurrence of sympatric species. We investigated how several small mammal species share a seasonally flooded meadow of limited size, testing predictions (P1) that herbivore, granivore, insectivore, and omnivore species are separated in time (dominant in different years), (P2) that sympatric species undergo isotopic partitioning, and (P3) that there are intraspecific differences in diet. Stable carbon and nitrogen isotope signatures in the hair of seven synantropic shrew, vole, and mice species were used as a proxy for their diet. We found that the three most abundant species in eight of the nine years were from different diet groups. However, based on the number of species in the functional groups, the state of small mammal community was considered unfavored in five out of the nine investigation years. In years with the greatest dominance of Apodemus agrarius, the small mammal community was characterized by decreased diversity and Micromys minutus was either in low abundance or absent. In 2014 and 2016, years of low abundance or absence of M. oeconomus, M. agrestis, and M. glareolus were both recorded in high numbers. Differences in the isotopic signatures of the three most abundant small mammal species in the community were clearly expressed and core areas in the isotopic space were separated, showing their dependence on different dietary resources. Intraspecific dietary separation between young and adult animals was observed only in M. oeconomus. Thus, the high species diversity of small mammals and the formation of their community in this investigated flooded meadow are maintained by isotopic partitioning (segregation in dietary space) and by changes in their number over time (shifting dominance).

The marine fish assemblage of the East Kleinemonde Estuary over 20 years: Declining abundance and nursery function?

Marine fishes in the intermittently open East Kleinemonde Estuary, South Africa, were sampled using seine nets over a twenty year period between 1994 and 2014. This dataset was analysed after 11 years (1994–2005), and represented one of the few medium-term studies of fish communities in an intermittently open estuary. A further nine years of sampling (2006–2014) allowed us to revisit the fish community and to gauge if 20 years of data provides more insight into long-term trends in the fish community. The overall species abundance follows a cyclical pattern with two to three years of low abundance and two to three years of high abundance. This pattern broke down in the last five years of the study, with a decline in species richness and consistently low catches of almost all marine species recorded. This may have been as a result of a reduction in the area of submerged macrophytes in the estuary, as well as an increase in the frequency of mouth opening and subsequent decrease in available water area.

Movement patterns determine the availability of Argentine shortfin squid Illex argentinus to fisheries

In this study, we found that dynamic movements of squid in its feeding phase have profound effects on its abundance and thus affect its availability to fisheries. The Argentine shortfin squid Illex argentinus is one of the world’s important economic invertebrate species, with annual fisheries production exceeding one million tons in some years, but the value drastically fluctuates by more than 20-fold. Using catch per unit effort (CPUE) data from the Taiwanese squid jigging fleets during the years 2004–2013, we evaluated its variability in abundance, and the most possible relationships to migration patterns. Results from the statistical analysis using a generalized additive model (GAM) showed that within a specific year, the CPUE was linearly related to latitude (between 46°S and 50°S) and sea surface temperature (SST; of between 10 and 14°C) components, corresponding to the squid’s dynamic movement during a fishing season. Two distinctive movement patterns for low- and high-abundance years were recognized. In low-abundance years (2004 and 2010), negative cross-correlations between the CPUE and the latitudinal component of the fishing position were found in a time-lagged analyses, suggesting a decoupling of the CPUE from latitude; while in high-abundance years (2007 and 2013), a positive correlation was found, signifying a strong coupling between the CPUE and latitude. A spectral analysis identified one significant long-period (low frequency) coherency between the CPUE and the latitudinal component in 2007, while no significant coherency was found in 2004, suggesting a feasible matchup of squid abundance and its southern feeding movement for an expected high annual production.

Diet overlap between juvenile flatfish and the invasive round goby in the central Baltic Sea

The present study offers a comprehensive analysis of changes in the abundance and diet composition of juvenile flounder (Platichthys flesus) and turbot (Psetta maxima), along with other dominant coastal fish species, before and after the establishment of the alien round goby off an exposed stretch of coast in the eastern Baltic Sea.In the study area, the round goby (Neogobius melanostomus) was recorded for the first time in 2009. After a few years of low abundance, a sharp increase in the population occurred. After the round goby invasion, flatfish juveniles exhibited an increased diet overlap with other species and had a lower feeding success, reflecting an increase in resource competition. For juvenile turbot, the increase was mainly caused by the round goby, while for flounder it was due to both the round goby and the lesser sandeel (Ammodytes tobianus).Juvenile turbot, whose dominant food item before the round goby establishment had been mysids, shifted their diet towards Crangon crangon, reflecting a decrease in mysid abundance by three orders of magnitude and a concurrent doubling in C. crangon abundance in the habitat. At the same time a significant decrease in turbot recruitment was observed.Juvenile flounder had the widest food spectrum of the studied species. When the availability of the primary food item, Bathyporeia pilosa, decreased, flounder juveniles adapted by increasing the share of zooplankton in their diets. No changes in flounder feeding success and recruitment were observed. However, the recruitment estimates of flounder and turbot show an increasing co-variation after the round goby invasion, suggesting that recruitment of the species may currently be regulated by processes in the common nursery habitat.

Climatic and economic drivers of the Bering Sea walleye pollock (Theragra chalcogramma) fishery: implications for the future

This paper illustrates how climate, management, and economic drivers of a fishery interact to affect fishing. Retrospective data from the Bering Sea walleye pollock (Theragra chalcogramma) catcher–processer fishery were used to model the impact of climate on spatial and temporal variation in catch and fishing locations and make inferences about harvester behavior in a warmer climate. Models based on Intergovernmental Panel on Climate Change scenarios predict a 40% decrease in sea ice by 2050, resulting in warmer Bering Sea temperatures. We find that differences in the value of catch result in disparate behavior between winter and summer seasons. In winter, warm temperatures and high abundances drive intensive effort early in the season to harvest earlier-maturing roe. In summer, warmer ocean temperatures were associated with lower catch rates and approximately 4% less fishing in the northern fishing grounds, contrary to expectations derived from climate-envelope-type models that suggest fisheries will follow fish poleward. Production-related spatial price differences affected the effort distribution by a similar magnitude. However, warm, low-abundance years have not been historically observed, increasing uncertainty about future fishing conditions. Overall, annual variation in ocean temperatures and economic factors has thus far been more significant than long-term climate change-related shifts in the fishery's distribution of effort.

Density-dependent regulation of year-class strength in age-0 juvenile striped bass (Morone saxatilis)

Abundance of age-0 striped bass ( Morone saxatilis ) exhibits 50-fold variability in Chesapeake Bay. Processes that act to reduce and thus regulate this variability were investigated. The potential for density-dependent regulation of growth and mortality in the early juvenile stage and its causes were investigated. Data from multiple seine and trawl surveys in upper Chesapeake Bay and tributaries were analyzed to construct growth and mortality indices having a high degree of spatial and temporal resolution. Age-0 mean lengths in September were inversely related to density, ranging from 67.8 mm in 1994, when mean density was 0.036·m–2, to 104.5 mm in 1992, when mean density was 0.003·m–2. Except for the Potomac River, evidence for density-dependent growth was consistent across subpopulations. Bioenergetics modeling indicated that prey consumption was limiting except in low-abundance years. Mortality increased with respect to abundance and also was density-dependent. The significant interaction between age-0 juvenile length in September and subsequent winter temperature on mortality indicated that density-dependent growth leads to size-selective overwinter mortality. A statistical model including age-0 abundances, age-0 lengths, and winter temperature explained a substantial fraction of variability and the mechanisms for regulation of striped bass recruitment.

Observing change in a North Sea benthic system: A 33 year time series

Since 1972 the macro-benthic infauna at station M1, 10.5 km off the Northumberland coast (central western North Sea), have been sampled by grabbing each March and September. The data series now includes over 500 taxa from 327 genera. During the 1970s the system showed a regular alternation of high and low abundance years and this was interpreted as evidence of density dependence, when this broke down in the 1980s winter temperature appeared to be involved. After 20 years of data collection analysis of the time series, showed that variability had increased and that there was a very strong link to phytoplankton production in the overlying water. The latest analyses show that this relationship has now also disappeared. The number of genera represented in the system has increased over time while total productivity has not altered significantly. Multivariate ordinations also show changes through time with breakpoints occurring in the early 1980s and early 1990s. The coastal benthos of this region shows decadal scale and longer term changes in biological composition and ecological function. The system is certainly influenced by climatic variation, environmental conditions (winter temperature) and carbon flux to the benthos. Over the 33 year time series none of these appears to have a dominant effect, rather different factors seem to dominate at different times. The extent to which this is due to a biological ‘system memory’ imposed by good recruitments of long lived taxa would warrant further study.

Brent goose colonies near snowy owls: Internest distances in relation to abundance of lemmings and arctic foxes

Brent goose colonies around snowy owl nests have been studied near Medusa Bay (73°21′ N, 80°32′ E) and in the lower reaches of the Uboinaya River (73°37′N, 82°10′E), the northwestern Taimyr Peninsula, from 1999 to 2006. All brent nests within 680 m from an owl nest have been regarded as an individual colony. The results show that the area of the colony is always larger than the protected area around the owl nest. In years of low abundance of lemmings, brent geese nest generally closer to the owl nest than in years of high abundance. When arctic foxes are abundant, however, brent geese nest significantly closer to owls than when the foxes are scarce, irrespective of lemming abundance. The mechanism of brent colony formation around owl nests is based on a number of stimuli.

Reproduction of the least weasel in captivity: basic observations and the influence of food availability

The least weaselMustela nivalis nivalis Linnaeus, 1766 is unique among carnivores because of its small body size and capacity for fast reproduction. It has been suggested to be the main agent for maintaining cyclic fluctuations in northern vole populations, largely based on its high reproductive potential and dependence on small rodents. This study describes basic reproductive data on the least weasel obtained during a captive breeding program. In the experiments, food availability was manipulated at the onset of breeding. Altogether, 65 litters were born during the 5-year study. The mean litter size was 5.1 (in 53 litters of known litter size), the most common litter size being six. The sex ratio of weaned young was not biased from 1∶1. The median date of birth was June 4. Food manipulations did not affect mating frequency suggesting that the observation of failure in breeding in years of low abundance of small mammals is due to mortality of embryos or young before weaning, but not due to avoidance of breeding at low food availability. In this respect, weasels differ from other main vole predators, owls and raptors, which often skip breeding if small mammals are scarce.

The potential use of environmental information to manage squid stocks

Most commercially exploited squid species have short life cycles and stocks composed of recruits from a single cohort, the size of which is unknown prior to the fishing season. Recent studies suggest that strong environment–recruitment relationships may exist for a number of squid stocks. Using simulation models based on Falkland Island Patagonian squid (Loligo gahi), the recruit abundance of which is predicted by sea-surface temperature, we propose a method for using predictive relationships in the management of squid populations. We compare a management strategy based on recruitment prediction with historical data from the fishery, which was managed in the absence of these predictions. Our results suggest that varying effort on the basis of an environmental correlate of recruitment can reduce the risk of not meeting conservation targets while increasing yield. Effort has to be reduced in years of low abundance but licensing additional effort in years of high abundance increases long-term average catches. Even if effort levels were not allowed to vary by more than 50% between years, a management strategy for L. gahi based on prediction would have resulted in higher average catches and a reduced probability of the stock biomass falling below a notional conservation limit.

The influence of a power station on the survival of juvenile sea bass in an estuarine nursery area

Trawl surveys in the estuary of the River Medway in the autumns of 1987–1991 have shown that the distribution of juvenile sea bass Dicentrarchus labrax is strongly associated with the warm‐water outflow from Kingsnorth power station. In years of low abundance of first‐year bass, very few were caught outside the warm‐water discharge channel, whereas proportionately more fish of abundant year classes occupied the main river. About 15% of the available juvenile bass population died on the cooling‐water intake screens at Kingsworth in the autumn and winter of 1987 and 1988. However, growth and survival of first‐year bass in the Medway Estuary may be enhanced by the power station's warm‐water effluent, such that overwinter mortality due to inadequate nutritional reserves and low temperatures may be considerably reduced.

Behavioural Responses of Coyotes and Lynx to the Snowshoe Hare Cycle

Coyotes and lynx are the two most important mammalian predators of snowshoe hares throughout much of the North American boreal forest. Populations of hares cycle in abundance. with peaks in density occurring every 8-11 years. We used snow-tracking to measure the diets, use of habitats, and hunting tactics of coyotes and lynx during a cyclic fluctuation of hare populations in the southwest Yukon. Our objective was to determine changes in foraging behaviour of the predators leading to functional responses to densities of hares. Coyotes and lynx both preferred snowshoe hares over available alternative prey at all phases of the cycle. Lynx switched to preying on red squirrels during the cyclic low and subsequent early increase. The pattern of changes in habitat use by coyotes and lynx paralleled that of snowshoe hares, and both concentrated their hunting activity in areas of high density of hares. Coyotes used more open cover to hunt voles during years of low abundance of hares and high numbers of small mammals. Lynx increasingly used ambush beds for hunting hares and red squirrels during the cyclic decline and low. Hunting success was not higher from beds. Lynx hunted in adult groups for the first time during the cyclic decline and low.

Annual and seasonal changes in diets of martens: evidence from stable isotope analysis.

Theory predicts that generalist predators will switch to alternative prey when preferred foods are not readily available. Studies on the feeding ecology of the American marten (Martes americana) throughout North America suggest that this mustelid is a generalist predator feeding largely on voles (Microtus sp.; Clethrionomys sp.). We investigated seasonal and annual changes in diets of martens in response to the changing abundance of small rodents (Peromyscus keeni, and Microtus longicaudus) on Chichagof Island, Southeast Alaska, using stable isotope analysis. We hypothesized that martens would feed primarily on small rodents during years with high abundance of these prey species, whereas during years of low abundance of prey, martens would switch to feed primarily on the seasonally available carcasses of salmon. We also hypothesized that home-range location on the landscape (i.e., access to salmon streams) would determine the type of food consumed by martens, and martens feeding on preferred prey would exhibit better body condition than those feeding on other foods. We live-captured 75 martens repeatedly, from mid-February to mid-December 1992-1994. We also obtained marten carcasses from trappers during late autumn 1991 and 1992, from which we randomly sub-sampled 165 individuals. Using stable isotope ratios and a multiple-source mixing model, we inferred that salmon carcasses composed a large portion of the diet of martens in autumn during years of low abundance of rodents (1991 and 1992). When small rodents were available in high numbers (1993 and 1994), they composed the bulk of the diet of martens in autumn, despite salmon carcasses being equally available in all years. Selection for small rodents occurred only in seasons in which abundance of small rodents was low. Logistic regression revealed that individuals with access to salmon streams were more likely to incorporate salmon carcasses in their diet during years of low abundance of small rodents. Using stable isotope analysis on repeated samples from the same individuals, we explored some of the factors underlying feeding habits of individuals under variable ecological conditions. We were unable to demonstrate that body weights of live-captured male and female martens differed significantly between individuals feeding on marine-derived or terrestrial diets. Therefore, martens, as true generalist predators, switched to alternative prey when their principal food was not readily available on a seasonal or annual basis. Although salmon carcasses were not a preferred food for martens, they provided a suitable alternative to maintain body condition during years when small rodents were not readily available.

Population cycles in the Varied Thrush (Ixoreus naevius)

The Varied Thrush (Ixoreus naevius), an endemic species of the Pacific Northwest of North America, shows dramatic year-to-year changes in wintering abundance. These changes have not, however, been systematically examined using standardized data. Using Project FeederWatch data from 1988–1989 to 1994–1995 we found that Varied Thrushes showed a biennial cyclic change in abundance within most of their normal wintering range. This cyclic pattern was also evident in Christmas Bird Count and Breeding Bird Survey data from the same time period. There was no difference in the extent or location of the winter range or in the distribution of elevations of Varied Thrush records between years of high and low abundance. More feeders had single Varied Thrushes in years of high abundance than in years of low abundance. Longer term (1969–1970 to 1994–1995) Christmas Bird Count and Breeding Bird Survey data showed a more variable pattern, with abundance peaks every 2–3 years. The number of vagrant Varied Thrushes from eastern North America reported in Audubon Field Notes winter season reports was not correlated with abundance in the normal wintering and breeding areas but was correlated with the number of Varied Thrushes tallied on southern California Christmas Bird Counts. These results suggest that patterns of vagrancy in Varied Thrushes are largely independent of population changes within the normal wintering area.

Length–Weight Relationships for Short-Finned Squid in Newfoundland and the Effect of Diet on Condition and Growth

Relationships of growth in weight to mantle length were examined for short-finned squid Illex illecebrosus collected near Holyrood, Newfoundland, from 1964 to 1985. For lengths typically encountered in the commercial fishery, males generally were significantly heavier than females at a given length. Fish appeared to be the most suitable prey type for promoting growth of inshore short-finned squid, whereas crustaceans appeared to be the least suitable prey type in that respect. Relatively high weight at length during July in years of high short-finned squid abundance was related to a high level of predation on fish. Seasonal change in condition was also directly related to prominence offish in the diet. Incidence of cannibalism increased dramatically after July in years of high short-finned squid abundance as predation on fish declined sharply. In contrast, there were no seasonal changes in the diet during years of low abundance. In such years, the diet was mixed throughout the season and cannibalism remained at relatively low levels.