Decline In Relative Abundance Research Articles

Novel MicroRNAs Differentially Expressed during Aging in the Mouse Brain

MicroRNAs (miRNAs) are endogenous small RNA molecules that regulate gene expression post-transcriptionally. Work in Caenorhabditis elegans has shown that specific miRNAs function in lifespan regulation and in a variety of age-associated pathways, but the roles of miRNAs in the aging of vertebrates are not well understood. We examined the expression of small RNAs in whole brains of young and old mice by deep sequencing and report here on the expression of 558 known miRNAs and identification of 41 novel miRNAs. Of these miRNAs, 75 known and 18 novel miRNAs exhibit greater than 2.0-fold expression changes. The majority of expressed miRNAs in our study decline in relative abundance in the aged brain, in agreement with trends observed in other miRNA studies in aging tissues and organisms. Target prediction analysis suggests that many of our novel aging-associated miRNAs target genes in the insulin signaling pathway, a central node of aging-associated genetic networks. These novel miRNAs may thereby regulate aging-related functions in the brain. Since many mouse miRNAs are conserved in humans, the aging-affected brain miRNAs we report here may represent novel regulatory genes that also function during aging in the human brain.

Slow recovery of previously depleted demersal fish at the South Shetland Islands, 1983–2010

Following seals and baleen whales prior to the 1970s, demersal fish stocks were depleted off the South Shetland Islands by intensive industrial fishing during the late 1970s to early 1980s. Little has been reported since about how these stocks have fared, after international agreement closed this fishery in 1990. We report changes in size and abundance of the commercially exploited Notothenia rossii and Gobionotothen gibberifrons relative to the ecologically similar but unexploited Notothenia coriiceps at Potter Cove, South Shetland Islands, over a 28-yr period, 1983–2010. N. rossii abundance declined from 1983 to 1991, and an increase in mean size during 1983–1984 is consistent with weak cohorts during preceding years. Modal age changed from 2–3 to 6–7 yr. Length data of G. gibberifrons, available from 1986, exhibited a similar pattern, showing a decrease until 1991–1992. After a period of relative stability (1992–1994), a sharp increase in length and a continued decline in relative abundance indicated low recruitment. The length–frequency distribution of unexploited N. coriiceps throughout the whole period showed no change in modal size or mean length of the fish. We relate these patterns to the fishery and suggest that a further two decades will lapse before these stocks recover. Using the South Shetland fisheries as an example, current management rules for Southern Ocean fisheries, deemed to be precautionary and disallowing depletion beyond which a stock can recover in 2–3 decades, may be unrealistic in an ocean profoundly altered by numerous stock depletions and rapid climate change.

Conservation cornerstones: Capitalising on the endeavours of long-term monitoring projects

Ecological monitoring is widely used to measure change through time in ecosystems. The current extinction crisis has resulted in a wealth of monitoring programs focussed on tracking the status of threatened species, and the perceived importance of monitoring has seen it become the cornerstone of many biodiversity conservation programs. However, many monitoring programs fail to produce useful outcomes due to inherent flaws. Here we use a monitoring program from south-eastern Australia as a case study to illustrate the potential of such endeavours. The threatened carnivorous marsupial, the brush-tailed phascogale (Phascogale tapoatafa), has been monitored at various locations between 2000 and 2010. We present strong evidence for a decline in relative abundance during this period, and also describe relationships with environmental variables. These results provide insights likely to be valuable in guiding future management of the species. In the absence of the monitoring program, informed management would not be possible. While early detection of population declines is important, knowledge of the processes driving such declines is required for effective intervention. We argue that monitoring programs will be most effective as a tool for enhanced conservation management if they test specific hypotheses relating to changes in population trajectories. Greater emphasis should be placed on rigorous statistical analysis of monitoring datasets in order to capitalise on the resources devoted to monitoring activities. Many datasets are likely to exist for which careful analysis of results would have benefits for determining management directions.

Retrospective Evaluation of the Effects of Human Disturbance and Goldfish Introduction on Endangered Pahrump Poolfish

Life history data for Pahrump poolfish (Empetrichthys latos latos) collected from 1937 to 1975 were organized and analyzed to improve our understanding of changes caused by human habitat disruption and introduced goldfish (Carassius auratus) at Manse Spring, Nevada. Pahrump poolfish twice demonstrated their ability to recover numerically from a population crash. The first crash followed a November 1961 introduction of a few goldfish and sub- sequent removal of vegetation by local ranch children intent on turning Manse Spring into a swimming hole. The sec- ond crash followed another major habitat disturbance resulting from an unsuccessful attempt to eradicate goldfish in July 1967. Each crash (1962-1963 and 1967-1968) reduced the poolfish population to fewer than 50 adults and was fol- lowed by a population recovery to more than 1000. Following the first habitat disruption, changes in poolfish population structure and poolfish diet were observed. These changes caused an increased mortality rate, resulting in disappearance or decline in relative abundance of larger poolfish size classes with a commensurate reduction in production of mature eggs by the population. We detected no additional changes in life history characteristics that could be associated with the second population crash and recovery. We concluded that removal of vegetation in the summer of 1962 and the more extreme habitat disruption and fish handling in June-July 1967 were human-induced pulse disturbances resulting in poolfish population crashes. We also concluded that the changes in poolfish life history characteristics resulted princi- pally from press disturbances attributable to goldfish.

Long-term change in an assemblage of North American bats: are eastern red bats declining

We examined changes in the assemblage of bats in southern Lower Michigan, USA, using results of paired netting surveys conducted with similar techniques but separated by 12–26 years. Species diversity declined by 18–35% and evenness decreased by 0–35% throughout the region and in two specific areas. Changes in diversity and evenness were attributed primarily to decreases of 44% or more in relative abundance of the foliage-roosting eastern red bat (Lasiurus borealis). Number of L. borealis captured per net-night decreased 52–85%. The decline in relative abundance of L. borealis suggested by mist netting was supported by a 10-fold decrease over 38 years in the proportion of L. borealis that were tested for rabies by the state health laboratory. The apparent decline in L. borealis is especially alarming in light of the recent upsurge in use of wind power and the large number of L. borealis that are killed at such developments. We recommend that other previously completed surveys in eastern North America be duplicated, as one way of helping to confirm or refute the trend that we have identified.

Detecting declines in catch rates of diverse trawl bycatch species, and implications for monitoring

Trawl fisheries around the world are being pressured to demonstrate that their impacts on both the target and bycatch species are sustainable in the long-term. However, the effectiveness of sampling catches to assess the viability of populations of non-targeted species is rarely examined. We estimated the levels of fishery-dependent sampling effort required to detect declines in catch rates of prawn trawl bycatch from 95 commercial trawls in two regions of Australia's Northern Prawn Fishery. We explore a range of possible monitoring options including combining different sampling intensities, time-frames and levels of statistical power. Poisson and negative binomial models were used to determine the number of trawls required to detect a range of declines. We found that between 15,536 and 24,933 trawls, depending on the region, would be required to detect a 20% decline in catch rates of the rarest taxa (<0.1 individuals h −1), assuming a power of 90% and a level of significance of 5%. Assuming a lower detection power (70%), trawl numbers would drop to 9126 and 14,646, respectively. Using a model of a constant decline in relative abundance (over 5 years), data accumulated from modest-sized annual surveys (52 and 43 trawls in two regions) would provide increasing power to detect changes in catch rates. After 3 years, surveys of this size (and power of 70%), could detect declines of 99.9% in 72–81% of taxa, declines of 50% in 34–43% of taxa, and declines of 20% in 20–34% of taxa, depending on the region. After 5 years, the power to detect declines of 50% had increased to cover 43–72% of taxa, and declines of 20% in 34–43% of taxa. Our results indicate that the power to detect even quite large declines in catch rates of rarely caught species would only be possible after some years of modest-sized annual surveys.

Decline in Relative Abundance of Bottlenose Dolphins Exposed to Long‐Term Disturbance

Studies evaluating effects of human activity on wildlife typically emphasize short-term behavioral responses from which it is difficult to infer biological significance or formulate plans to mitigate harmful impacts. Based on decades of detailed behavioral records, we evaluated long-term impacts of vessel activity on bottlenose dolphins (Tursiops sp.) in Shark Bay, Australia. We compared dolphin abundance within adjacent 36-km2 tourism and control sites, over three consecutive 4.5-year periods wherein research activity was relatively constant but tourism levels increased from zero, to one, to two dolphin-watching operators. A nonlinear logistic model demonstrated that there was no difference in dolphin abundance between periods with no tourism and periods in which one operator offered tours. As the number of tour operators increased to two, there was a significant average decline in dolphin abundance (14.9%; 95% CI=-20.8 to -8.23), approximating a decline of one per seven individuals. Concurrently, within the control site, the average increase in dolphin abundance was not significant (8.5%; 95% CI=-4.0 to +16.7). Given the substantially greater presence and proximity of tour vessels to dolphins relative to research vessels, tour-vessel activity contributed more to declining dolphin numbers within the tourism site than research vessels. Although this trend may not jeopardize the large, genetically diverse dolphin population of Shark Bay, the decline is unlikely to be sustainable for local dolphin tourism. A similar decline would be devastating for small, closed, resident, or endangered cetacean populations. The substantial effect of tour vessels on dolphin abundance in a region of low-level tourism calls into question the presumption that dolphin-watching tourism is benign.

Effects of urbanization and land use on fish communities in Valley Creek watershed, Chester County, Pennsylvania

Valley Creek watershed, located in southeastern Pennsylvania, is a small, fourth-order stream that empties into the Schuylkill River at Valley Forge National Historic Park, thirty-five kilometers northwest of Philadelphia. The 64 km2 watershed has been under extreme urbanization pressure over the past 30 years, resulting in rapidly increasing impervious surface cover and decreasing open space. The purpose of this study was to document some of the effects of urbanization on fish assemblages by quantifying the fish communities at fifteen sites throughout the watershed. Long-term effects of continued urbanization were identified, as data from the present study were compared to similar work completed nearly ten years earlier. There has been a shift in species composition from intolerant, coldwater species to more tolerant, eurythermal species. Currently, Valley Creek is supporting a naturally reproducing population of brown trout, but there has been a marked decline in relative abundance and range since 1993. Increased stream temperature from urban run-off is one of the primary issues in Valley Creek. Species composition was unique at each of the 15 stations owing to the effect of local land use in each station’;s drainage area. Fish assemblages revealed a patchy, non-continuous pattern of fish distribution.

Effects of recreational and commercial fishing on blue sharks (Prionace glauca) in Atlantic Canada, with inferences on the North Atlantic population

The nominal catch of blue sharks (Prionace glauca) reported for the Canadian Atlantic grossly underestimates the annual catch mortality of about 1000 tonnes (t), making blue sharks the most frequently caught large shark in Canadian waters. Although blue sharks accounted for 99% of all sharks landed at recreational shark fishing tournaments, tournament catches accounted for only 3% of total fishing mortality. Standardized catch rate indices suggested a decline in blue shark abundance of about 5%–6%·year–1 since 1995. An increased mortality rate in recent years was suggested by a decline in the median size of blue sharks in the commercial catch. Two independent calculations suggest that North Atlantic catches exceeded 100 000 t, with catch mortalities ranging between 26 000 and 37 000 t. Because tagging studies indicated that blue sharks are highly migratory with a single population in the North Atlantic, the Canadian contribution to overall population mortality accounts for only 2% of the total. The fact that blue shark populations are relatively productive and resilient may help explain their persistence in the face of high international catch mortality and a decline in relative abundance.

Development and evaluation of catch per unit effort indices for southern blue whiting (Micromesistius australis) on the Campbell Island Rise, New Zealand

Abstract This paper develops standardized commercial cpue indices for a highly aggregated spawning fishery in New Zealand waters, and verifies the indices using fishery-independent data. Indices were calculated for all vessels using three different measures of effort, and for vessel subsets based on processing type (surimi and dressed), and relative experience in the fishery. Trends in cpue were consistent with trends in fishery-independent acoustic surveys, age composition of the commercial catch, and recent stock assessment results. In particular, the cpue indices tracked the more than fourfold increase in abundance from 1993 to 1996 associated with the recruitment of the strong 1991 year class, and the decline in relative abundance as this year class was fished down. Despite this being a highly aggregated spawning fishery, there was little evidence for hyperstability. There were also significant differences in fishing strategies of the fleets between periods of high and low fish abundance.

Paleolimnological evidence of change in a shallow, hypereutrophic lake: Upper Klamath Lake, Oregon, USA

Sediment cores were collected from Upper Klamath Lake in October, 1998 and analyzed for 210Pb, 14C, 15N, N, P, C, Ti, Al, diatoms, Pediastrum, and cyanobacterial akinetes. These results were used to reconstruct changes in water quality in Upper Klamath Lake over the last 150 years. The results showed that there was substantial mixing of the upper 10 cm of sediment, representing the previous 20 to 30 years. However, below that, 210Pb activity declined monotonically, allowing reasonable dating for the period from about 1850 to 1970. The sediment accumulation rates (SAR) showed a substantial increase in the 20th century. The increase in SAR corresponded with increases in erosional input from the watershed as represented by the increases in sediment concentrations of Ti and Al. The upper 20 cm of sediment, representing the last 150 years, also showed increases in C, N, P, and 15N. The increases in nutrient concentrations may be affected to various degrees by diagenetic reactions within the sediments, although the changes in concentrations also were marked by changes in the N:P ratio and in a qualitative change in the source of N as reflected in increasing δ15N. The diatoms showed modest changes in the 20th century, with increases in Asterionella formosa, Stephanodiscus hantzschii, and S. parvus. Pediastrum, a green alga, was well-preserved in the sediments and exhibited a sharp decline in relative abundance in the upper sediments. Total cyanobacteria, as represented by preserved akinetes, exhibited only minor changes in the last 1000 years. However, Aphanizomenon flos-aquae, a taxon which was formerly not present in the lake 150 years ago, but that now dominates the summer phytoplankton, has shown major increases over the past 100 years. The changes in sediment composition are consistent with activities including timber harvest, drainage of wetlands, and agricultural activities associated with livestock grazing, irrigated cropland, and hydrologic modifications.

Bathymetric distribution of some benthic and benthopelagic species attracted to baited cameras and traps in the deep eastern Mediterranean

A series of baited camera and trap experiments in the eastern Mediterranean Sea between 1500 and 4264 m depth attracted a variety of opportunistic scavengers, with species composition changing with increasing depth. At the shallower stations (1500 to 1800 m), decapod crustaceans and fishes, dominated by elasmobranchs such as Hexanchus griseus, were attracted to and actively consumed the bait. Some of these species were observed at depths exceeding their previously reported ranges. This was believed to be a result of the absence of deep-water scavengers from the adjacent Atlantic due to dispersal barriers and elevated temperatures at depth. The diversity of bait-attending fauna declined with increasing depth. Elasmobranchs were not observed below 2500 m, and below 4000 m only the caridean shrimp Acanthephyra eximia and the macrourid Chalinura mediterranea were present; at this latter depth, bait consumption was negligible. This shift in species composition was reflected in changes in first arrival times. Increasing first arrival times of H. griseus suggested a decline in relative abundance from 1500 to 2500 m, whilst those of C. mediterranea indicated an increase in relative abundance from 1800 to 4264 m.

Decline of disjunct green salamander ( Aneides aeneus) populations in the southern Appalachians

Coincident with other amphibians around the world Aneides aeneus, a terrestrial plethodontid salamander, suffered a population collapse in a disjunct portion of its range in the mid–late 1970s. Long-term monitoring of seven historical green salamander populations throughout the 1990s showed a 98% decline in relative abundance since 1970. Three out of six populations first discovered in 1991 also crashed in 1996–1997. The synchronized suddenness of the declines, their region-wide impact, and effects on both small and larger populations, suggest the role of a novel agent of mortality beginning in the mid–late 1970s. Acting alone, but more likely in concert, habitat loss, overcollecting, epidemic disease and climate change could account for this region-wide decline.

Integrated stratigraphy of the Waitakian‐Otaian Stage boundary stratotype, Early Miocene, New Zealand

The base of the type section of the Otaian Stage at Bluecliffs, South Canterbury, is recognised as the stratotype for the boundary between the Waitakian and Otaian Stages. Principal problems with the boundary are the restriction of existing bioevent proxies to shelf and upper slope environments and its uncertain age. These topics are addressed by a multidisplinary study of a 125 m section about the boundary, which examines its lithostratigraphy, depositional setting, biostratigraphy, correlation, and geochronology. The lower siltstone lithofacies (0–38.5 m) was deposited at upper bathyal depths (200–600 m) in a marginal basin which was partially sheltered from fully oceanic circulation by a submarine high and islands. The site was covered by cool‐temperate water and was probably adjacent to the Subtropical Convergence. This unit is succeeded by the banded lithofacies (38.5–106 m) and the upper siltstone lithofacies (basal 19 m studied). Paleodepth probably declined up‐sequence, but deposition at shelf depths is not definitely indicated. A cyclic pattern of abundance spikes in benthic and planktonic foraminifera commences 9 m above base and extends to 73 m in the banded lithofacies. Oxygen isotope excursions (up to 2.08%o) in Euuvigerina miozea and Cibicides novozelandicus are greatest within the interval containing the abundance spikes. The stage boundary occurs in the banded lithofacies at the highest abundance spike (73 m). Although condensed intervals might affect the completeness of the section, they are not associated with sedimentary discontinuities, and we consider that the section is suitable as a biostratigraphic reference. Spores, pollens, dinoflagellates, calcareous nannofossils, foraminifera, bryozoans, and ostracods are preserved near the boundary, but molluscs principally occur higher, in the shallower upper siltstone lithofacies. Siliceous microfossils are rare. There is considerable scope for further biostratigraphic research. The primary event marking the boundary at 73 m is the appearance of the benthic foraminifer Ehrenbergina marwicki. This is a distinctive and widely distributed event but is restricted to shelf and upper bathyal environments. Supplementary events in planktonic foraminifera and calcareous nannofossils were researched. Highest occurrences of Globigerina brazieri and G. euapertura are recorded at 47 and 58 m. There is a marked decline in relative abundance of Paragloborotalia spp. at 62 m. Helicosphaera carteri becomes more abundant than H. euphratis between 56 and 87 m. These events are not exact proxies for the boundary but they may usefully indicate proximity to it. They occur in the interval of prominent spikes in foraminiferal abundance. The Waitakian‐Otaian boundary is dated at 21.7 Ma by strontium isotopes. Stable primary remanence could not be determined in a pilot paleomagnetic study of Bluecliffs specimens. However, specimens trended towards reversed polarity, and remagnetisation great circle analysis will allow directions to be calculated in future collections.

Use of 16S-rRNA Based Techniques to Investigate the Ecological Succession of Microbial Populations in the Immature Lamb Rumen: Tracking of a Specific Strain of Inoculated Ruminococcus and Interactions with Other Microbial Populations in Vivo.

The establishment of microorganisms in the rumen is a critical step if rumen manipulation is to be accomplished by use of microbial inoculants. Microbial populations in the maturing rumen undergo successional changes and, while in a state of flux, provide a possible opportunity for the introduction of specific strains of bacteria. While the rumen of the young lamb was maturing, we measured changes in several microbial populations with 16S-rRNA specific oligonucleotides: Rumincoccus, Fibrobacter, eukaryotes, Gram-positive bacteria, the Bacteroides-Porphromonas-Prevotella group, and anaerobic rumen fungi. In this study we repeatedly dosed 15 lambs with approximately 3.4 x 10(8) to 0.8 x 10(9) Ruminococcus cells dose(-1), twice a week, for 7 wk from 23 d to 63 d of age. Of the five Ruminococcus strains dosed (R. albus SY3 and AR67, and R. flavefaciens Y1, LP9155, and AR72) the most specific primers (based on 16S rDNA) were obtained for strain SY3. There was an increase in the eukaryotic population during dosing, and it was hypothesized that protozoal predation contributed to the disappearance of strain SY3. At the end of dosing PCR amplification showed that SY3 were approximately 10(9) cells ml(-1), but decreased to below the detection limit of the PCR system (8.6 x 10(4) ml(-1)) within 28 d postdosing. These experiments showed that fibrolytic populations increased significantly (P < 0.1) above the controls during the dosing period and were elevated for several days postdosing. This suggests that dosing of highly fibrolytic bacteria makes more of the fiber available to other organisms able to degrade fiber, and in so doing increases the overall fibrolytic activity of the rumen. Examination of the succession of gram-positive bacteria and the Bacteroides-Porphromonas-Prevotella group showed a decline in relative abundance as the lambs matured.

Senescence-Related Changes in the Subcomplex Arrangement of the Major Light-Harvesting Chlorophyll a/b-Protein Complex of Photosystem II (LHCII) as Influenced by Cytokinin

Abstract The major light-harvesting chlorophyll a /b -protein complex of photosystem II (LHCII) from fresh barley leaves could be resolved by non-denaturing IEF into five trimeric subcomplexes designated 1 -5 in order of decrasing pi value. IEF-based analysis of PSII particles isolated from leaves in which the processes of senescence were induced by detachement and dark-incubation in the presence of water for 0 -8 days let us reveal that substantial rearrangements of LHCII organization took place throughout the course of senescence comprising a step-wise decline in relative abundance of subcomplexes 1 -3 (down to 0-58% of the initial abundance during 8 days of aging) and an increase in the relative abundance of the subcomplexes 4 and 5. Using SDS-PAGE and immunoblot analysis it was shown that the rearrangements were linked to the changes in the relative levels of LHCII apoproteins i.e. 26.7 and 25.6 kDa ones. The changes comprised the preferential disappearance of the 26.7 kDa polypeptide and an enrichment of 25.6 kD a one and most probably reflect the hetero­ geneity among LHCII apoproteins concerning their stability under the conditions of chi loss. Kinetin was able to repress the senescence-related rearrangements in LHCII subcomplex organization at late stages of aging (5 -8 days) by preventing over this time period the disappearance of 26.7 kD a polypeptide and the enrichement of 25.6 kDa one.

Evidence of a decline in beluga,Delphinapterus leucas, abundance off West Greenland

Aerial surveys of belugas, or white whales,Delphinapterus leucas, were conducted off West Greenland in March 1993 and 1994. These surveys were designed to permit comparisons with similar surveys in 1981, 1982, and 1991. Differences in methods between the surveys in the early 1980s and those in the 1990s should mean that the 1990s surveys were, if anything, more efficient than the 1980s surveys. Weighted linear regressions of strip-census estimates of the relative abundance of belugas showed a significant decline from 1981 to 1994. While annual variability in ice conditions and other unknown factors may have had some local or small-scale effects on the movements and distribution of belugas off West Greenland, it is unlikely that these effects would account for the apparent decline in relative abundance. The most reasonable conclusion is that the stock size has declined considerably during the past 13 years, perhaps by as much as 62%.

Identification of receptor tyrosine kinases in the rat testis.

Evidence of receptor/ligand interactions that regulate testis cell function was sought in order to broaden the current understanding of the molecular basis of testis cell function. Using reverse transcription and the polymerase chain reaction, we have obtained novel evidence for the expression of three mRNAs encoding receptor tyrosine kinases in the adult rat testis: the platelet-derived growth factor type A receptor (PDGF-RA), the basic fibroblast growth factor receptor (flg), and fetal liver kinase 1 (Flk-1). A 6.8 kb transcript encoding the PDGF-RA was observed in RNA prepared from testes of rats aged day 5 through adult, with a decline in relative abundance with increasing age after day 17. Analysis of mRNA from isolated cell preparations (day 21 Sertoli cells, adult Leydig cells, round spermatids, and primary spermatocytes) and testes depleted of specific cell types [ethane dimethane sulfonate (EDS)-treated and cryptorchid] indicated that the Leydig cell was the predominant source of this mRNA in the adult testis. The addition of PDGF-BB to cultures of highly purified adult rat Leydig cell preparations resulted in a 40% increase in LH-stimulated testosterone production, confirming a role for this growth factor in regulation of Leydig cell function. These data indicate that the Leydig cell is a principal site of action of PDGF in the testis.

An Index of the Relative Abundance of Wintering Belugas,Delphinapterus leucas, and Narwhals,Monodon monoceros, off West Greenland

Systematic aerial surveys of the wintering grounds of belugas, Delphinapterus leucas, and narwhals, Monodon monoceros, in southern Baffin Bay and northern Davis Strait were conducted in late winter of 1981, 1982, 1990, and 1991. Most belugas were found between 67°N and 69°N and none were seen more than 80 km off the coast of West Greenland. Compared with the surveys in 1981 and 1982, a decline in relative abundance of belugas along West Greenland was evident in 1991. This decline was significant at a probability level of 0.13 of the bootstrapped distribution of the combined abundance estimate. Pod sizes declined significantly between the 1981–82 and 1990–91 surveys. The variations in ice conditions between years did not seem to affect the distribution, clumping, or pod sizes of the belugas. Narwhals were widely distributed in the close pack ice offshore between 65°N and 72°N. Along the West Greenland coast, narwhals were primarily seen at the mouth of Disko Bay. No change in relative abundance or pod sizes could be detected for narwhals.

Evidence of predatory control of yellow perch (Perca flavescens) recruitment in Lake Erie, U.S.A.

Predation can be a major factor in recruitment success of yellow perch (Perca flavescens Mitchitl). Trawl catches of age‐0 yellow perch in western Lake Erie declined from 870·3 per trawling h in June to 3·3 per trawling h in late July 1988. Coincident with the decline in relative abundance of age‐0 yellow perch we found large numbers of age‐0 yellow perch in the stomachs of small walleyes (Stizostedion vitreum Mitchill). From this evidence we hypothesized that predation by walleyes may have caused the demise of the 1988 year‐class of yellow perch. We used a population and bioenergetics modelling approach to estimate the impact of walleye predation on the abundance of age‐0 yellow perch. Modelling showed that 6·8 × 109 age‐0 yellow perch that had attained 18 mm total length (t.l.), were eaten by small (age‐2 and younger) walleye from June through July 1988. We estimated that walleye ate 28·4–89·7% of the yellow perch reaching 18 mm t.l. during 1988. The majority of this predation (77% of total) was by the abundant age‐2 cohort of walleyes. We concluded that, even in a large system such as Lake Erie, predation can play a major role in structuring year‐class strength of yellow perch and, thus, management of percid fisheries should be conducted on a fish‐community basis.