Now showing items 21-40 of 495

• #### The significance of marine-derived biogenic nitrogen in anadromous Pacific salmon freshwater food webs

The natural abundance of the stable isotope ratios $\sp{15}$N/$\sp{14}$N and $\sp{13}$C/$\sp{12}$C expressed as $\delta\sp{15}$N and $\delta\sp{13}$C was used to trace biogenic nutrients delivered by returning adult anadromous Pacific salmon into freshwater systems. These systems were Sashin Creek, a rapidly flushing stream located on Baranof Island, southeastern Alaska and Iliamna Lake, the major sockeye salmon, Oncorhynchus nerka, nursery lake in the Kvichak River watershed, Bristol Bay, southwestern Alaska. Marine-derived nitrogen (MDN) was quantifiable by use of an isotope mixing model based on comparison of biota $\delta\sp{15}$N in areas used for spawning by anadromous salmon with salmon-free controls within the same watershed. Control periphyton (benthic primary producers) $\delta\sp{15}$N values $\sim$0 suggested that the control N pool was derived from N$\sb2$ fixation without significant recycling. In contrast, periphyton abundant in areas of intense spawning activity or carcass aggregation had $\delta\sp{15}$N $\sim$ +7. These two values were the basis for comparison of $\delta\sp{15}$N values of higher trophic level biota. A mixing model relating $\delta\sp{15}$N to MDN with trophic level was used to estimate consumer MDN through incorporation of a priori isotopic trophic enrichment factors established in the literature. Distinctive $\delta\sp{13}$C signatures along the Sashin Creek stream gradient and between Iliamna Lake littoral and limnetic production were used in concert with $\delta\sp{15}$N. Sashin Creek fishes reflected isotopic signatures of periphyton and thus production within the same stream section. Isotopic data suggested an overall importance of limnetic production in Iliamna Lake resident fish and juvenile sockeye salmon diets. Salmon eggs and emergent fry retaining the parental marine isotopic signature were distinguishable from autochthonous production derived from marine N, and appear to be a minor dietary component in both Sashin Creek or Iliamna Lake fishes. The proportion of MDN in resident fish N, including juvenile salmon after turnover of the natal N pool, was proportional to the escapement of spawners. Thus there is now direct evidence for a significant natural fertilization process: the flow of remineralized marine-derived biogenic nutrients from returning anadromous Pacific salmon into freshwater food webs.
• #### Thyroid hormone binding to brain nuclear extracts during smoltification in coho salmon

Salmon complete a metamorphosis called smoltification prior to entering salt water. Increased thyroid activity, olfactory imprinting, and chemical and structural changes in the brain are known to occur at this time. This study was undertaken to determine if triiodothyronine (T$\sb3$) binding to brain nuclear extracts changes during smoltification. During this investigation serum thyroxine (T$\sb4$) concentrations increased three fold during smoltification coincident with changes in coloration and morphology and surged again during downstream migration to six times presmolt concentrations. Using ultrafiltration assays, homologous displacement experiments of KCl extracts of recovered brain cell nuclei indicated that maximal binding capacity increased during smoltification and down-stream migration. The increase in receptor concentration lagged the increase in serum thyroxine by one week. Dissociation constants increased during smolt transformation but declined abruptly during down-stream migration. However, dissociation constants did not change during smoltification if nuclear extracts had been previously incubated at room temperature to remove endogenous ligand. Dissociation rate increased significantly, coincident with the increase in receptor concentration measured by homologous displacement. The maximal probable percent occupancy of available receptors increased from 60% before to greater than 95% during the smolt transformation climax. These results provide evidence that thyroid hormone receptors participate in brain development and olfactory imprinting in smolting salmon.
• #### Distribution and abundance of the Pacific razor clam, Siliqua patula (Dixon) on the eastside Cook Inlet beaches, Alaska

Three questions were asked about the population of the Pacific razor clam Siliqua patula (Dixon) on eastside Cook Inlet beaches: (1) can density be estimated by a three-stage stratified random sampling plan; (2) can age composition data be used for age-structured population estimation; (3) does substrate composition affect clam density? Field studies of Coho, Ninilchik and Clam Gulch beaches obtained precise density estimates for Clam Gulch beach only (coefficient of variation = 14.6%, 1988, and cv = 13.6%, 1989). A heavily exploited area of high density at Clam Gulch was resampled extensively in 1989 to determine if a significant harvest rate was detectable. No significant harvest rate was detected. A catch-at-age model was successfully applied to age-structured data, and estimates of abundance for ages 4 through 11+ in years 1977 to 1989 were generated. There is some evidence from substrate analyses that clams are found in higher abundance where grain sizes 0.125 to 0.400 mm predominate.
• #### Diel vertical migration by juvenile sockeye salmon and zooplankton in a stained and a glacial lake

Vertical distributions of both juvenile sockeye salmon (Oncorhynchus nerka) and zooplankton in a stained (Secchi depth 4.3 m) and a glacial (Secchi depth 0.2 m) lake in Alaska were measured at four times within a 24-h period. Stained-lake juveniles avoided surface during daylight, crossed large temperature gradients, and preyed upon cladoceran zooplankton >0.30 mm long. Glacial-lake juveniles preferred surface (<1.5 m deep) during the daylight, experienced little temperature change, and preyed upon copepod zooplankton >0.30 mm and terrestrial insects. Zooplankter depth distributions overlapped with day and night juvenile sockeye depth distributions in the glacial lake, but they overlapped only at night in the stained lake. The amplitude of juvenile sockeye vertical migration is positively correlated with euphotic zone depth. Sockeye vertical distribution is not related to absolute light intensity, zooplankton abundance or distribution, or thermal stratification. Most applicable are multiple factor explanations that incorporate predation risk and forage abundance for the juvenile sockeye.
• #### Prey consumption by juvenile salmonids on the Taku River, southeast Alaska

Stomach contents were collected from juvenile salmonids (genus Oncorhynchus and Salvelinus) from habitats on the Taku River in 1987. Differences were defined between groups of fry. A linear discriminant function (LDF) analysis was applied to prey frequencies grouped by species, habitat, and period. The analysis discriminated between: fish in beaver ponds; sockeye in side-slough sites and fish from other mainstem sites; and beaver ponds and mainstem sites. An exclusion experiment was established in a beaver pond. The diet of sockeye (O. nerka) and coho (O. kisutch) fry was sampled from allopatric and sympatric treatment enclosures. LDF analysis applied to prey categories assigned group membership between species, treatment, and period factors. A log-linear analysis yielded significant interaction effects between the treatment, habitat, and. period explanatory variables and the response, prey, confirming the influence of the presence of cogenerics on prey consumed.
• #### Separation of sockeye salmon stocks in east side Bristol Bay commercial harvests, 1983-1989

Even though the fishery along the east side of Bristol Bay is considered to be a terminal fishery, the fishery does harvest mixed stocks. When mixed stocks are harvested, catch allocation and run strength estimates become questionable. Using scale growth analyses, the stock compositions of the commercial catches were estimated for 1983-1985. Classification accuracies for the scale measurement models created for 1983-1985 ranged from approximately 70%-81% correct classification. The highest interception rates, 32%-46%, were estimated in the Egegik and Ugashik Districts in 1984 and 1985. Lower amounts of interception were estimated in 1983 for all districts. Interception rates for 1983-1985 were combined with the Alaska Department of Fish and Game results for 1986-1989 to look for trends in interception. With only seven years of data, little could be substantiated from regressions and graphical analysis.
• #### Seabirds at sea in relation to oceanography

This study investigated the macroscale distribution of seabirds in relation to oceanography in a neritic environment characterized by well-defined water masses (the northern Bering Sea) and an oceanic environment characterized by weaker differences between water masses (the northern North Pacific Ocean). In the northern Bering Sea, the total density (birds/km$\sp2)$ of all seabirds combined and densities and/or frequencies of occurrence of seven of nine species of seabirds that exhibited significant differences among water masses showed the strongest attraction to Anadyr Water. In general, attractions were second highest in Bering Shelf Water, third highest in Two-layered Water (Alaska Coastal Water overlying Bering Shelf Water), and lowest in Alaska Coastal Water. This pattern of seabird distributions reflected distributions of zooplankton biomass, which were highest in Anadyr Water and consisted of species that were large enough to be eaten directly by seabirds. Further, whereas copepods in Bering Shelf Water also are large, they are much smaller in Alaska Coastal Water and, thus, must pass through more trophic levels to fishes before the energy is directly accessible to seabirds. Consequently, zooplankton-based food webs dominated in Anadyr and Bering Shelf waters and fish-based food webs dominated in Two-layered and Alaska Coastal waters. In addition, seabirds concentrated near a strong, mesoscale thermal front between Bering Shelf and Alaska Coastal waters. In the northern North Pacific, assemblages of seabirds exhibited three main groupings, a "subarctic assemblage," a "transitional assemblage," and a "'subtropical/tropical assemblage." These assemblages matched those for zooplankton, squids, and fishes in the same vicinity, suggesting that there are geographically- and temporally-stable biological communities in the North Pacific that are associated with well-defined, persistent physical environments. The total density of all seabirds combined and densities and/or frequencies of occurrence of 13 of 16 species of seabirds that exhibited significant two-way ANOVAs exhibited primarily a water mass effect; only one species exhibited primarily a year effect, and two exhibited primarily an interaction (i.e., a change in habitat use between years).
• #### Forecasting catches of Pacific salmon in commercial fisheries of southeast Alaska

Data collections since 1911 and statistical methods from time series analysis are employed to forecast catches of pink, chum, coho, and sockeye salmon in Southeast Alaska. Knowledge of the spatial and temporal domains favored by Pacific salmon originating in Southeast Alaska is summarized to provide a basis for estimating environmental variation experienced by each species. Catches in northern, southern, and all of Southeast Alaska are forecast with univariate ARIMA, transfer function-noise (TFN), and vector ARMA models. Univariate models for catch in numbers and catch in weight yielded similar results for each species. Air and sea surface temperatures, freshwater discharge, and coastal upwelling enter TFN models for several species and areas. Environmental variables allow TFN models to explain a small amount of variation in the catches (average of 19%) above that explained by univariate models. Forecasts for most, but not all, species and areas are improved (average of 16%) by including environmental data in TFN models. Stock-recruit models with a parameter for density dependent mortality provide the best forecasts of pink salmon catch and are recommended for future forecasts. Winter air and sea surface temperatures enter stock-recruit models for pink salmon, and forecasts of catch and recruitment in northern and southern Southeast Alaska tend to oppose each other and cancel (1981-1985), which suggests that the salmon are caught in areas other than where they originated. Mean absolute percentage error (MAPE) for forecasts of pink salmon catch from stock-recruit models in Southeast Alaska, based on data for 1981-1990, is estimated at 49%, with first, second, and third quartiles of 10%, 23%, and 83%, respectively. Catches of Pacific salmon in Southeast Alaska are significantly correlated and are forecast jointly with good accuracy by vector ARMA models, except when effects believed to result from density dependent mortality are present in the data. Correlations indicate that coho salmon smolts might prey on young pink salmon. Also, recruitment of pink salmon in Southeast Alaska and British Columbia is correlated; regional environmental influences might thus affect catches in both areas. In Southeast Alaska, MAPE for forecasting coho and sockeye salmon catch with time series analysis is about 20%, and about 30% for chum salmon.
• #### Bitter crab disease studies: observations on seasonality, mortality, species susceptability and life history

Incidence and average intensity of Bitter Crab Disease (BCD) in Auke Bay Tanner crabs CChionoecetes bairdi) were significantly greater during June through September of both 1989 and 1990 then during October through May. BCD is a chronic but fatal disease; crabs did not develop immunity and often died from secondary bacterial and ciliate infections. Total mortality exceeded incidence and was not significantly different between summer and winter seasons or between years. BCD appears to be host specific: red king crabs (Paralithodes camtschaticus) and Dungeness crabs (Cancer maaister) did not contract BCD post-injection. BCD amoeboid stages consistently caused disease in Tanner crabs when injected into the hemocoel, while dinospore stages did not. Waterborne challenges did not cause disease. BCD parasites did not occur intracellularly, remaining within the hemal and vascular systems. Parasites exited the host via gills and possibly esophagus. The life cycle of BCD dinoflagellates outside their hosts remains incompletely described.
• #### On interannual variability and climate change in the north Pacific

Long term changes in the atmospheric and oceanic environment of the North Pacific were investigated for the period 1946-1991. A climatology of North Pacific wind stress curl was developed because of the relevance of changes in wind stress curl to both oceanic and atmospheric variability. The dominant scales of spatial and temporal wind stress curl variability were determined and examined within the context of observed changes in North Pacific air temperature, sea surface temperature (SST), sea ice cover, oceanic mass transport and the occurrence of blocking anticyclones. Relationships between these variables and indices of tropical Pacific variability were also determined on interannual time scales. During 1976-1988, phase relationships were very strong between long term mean anomalies of wind stress curl, SST, air temperature, sea ice cover, The Pacific North American index, the Southern Oscillation Index (SOI), and tropical Pacific SST. Long term mean anomalies of these parameters did not change sign during 1976-88. These strong phase relationships did not occur amongst these variables during any other period of the record. The 1976-1988 period is characterized by intensified storminess, the decreased occurrence of blocking anticyclones, and decreased sea ice cover in the subarctic North Pacific. Intensified atmospheric circulation also occurred in the western Pacific subtropical anticyclone. Anomalously low SST occurred across the central and western North Pacific during this period while anomalously high SST was present in the eastern North Pacific adjacent to North America. Changes in the sign of the long term mean anomalies of wind stress curl, central North Pacific SST and the SOI suggest that this climate regime ended or relaxed after 1988. After 1975, long term changes in anomalies of the Southern Oscillation Index, tropical Atlantic wind stress, Sahel rainfall, and Greenland Sea ice cover have characteristics similar to those observed in the North Pacific. It is suggested that the climate anomalies observed in the North Pacific during 1976-1988 occurred as part of a hemispheric or global scale climate regime.
• #### Injury, survival and growth of rainbow trout captured by electrofishing

Electrofishing injury studies in Arizona and Alaska revealed spinal injury rates of over 50% among large (>300 mm long) rainbow trout Oncorhynchus mykiss captured by electrofishing with pulsed direct current (PDC). My goal was to identify an alternative waveform that would efficiently capture large rainbow trout with injury rates less than 15%. Experiments in homogeneous and heterogeneous electrical fields tested six waveforms; lower injury rates resulted with DC (17%), CPS™ (8%), and 20-Hz PDC at 75% duty cycle (25%). In field experiments with these three waveforms, PDC and DC gave higher capture rates than CPS™. However, injury rate was 60% with 20-Hz PDC and highly variable (0-47%) with DC. Long-term mortality of rainbow trout shocked with 60-Hz PDC at 50% duty cycle was 35% after 203 days. I recommend DC as an alternative to PDC waveforms for relatively safe and efficient capture of large rainbow trout.
• #### Circulation variability in the Bering Sea

Sea surface height anomalies measured by the GEOSAT radar to develop an improved circulation scheme for the deep basin of the Bering Sea and to make inferences about the dynamics controlling the circulation. A conceptual model of the circulation in the Bering Sea is presented in three manuscripts. The first provides a description of a large eddy in the Alaskan Stream. This eddy is shown to influence circulation in the southern Aleutian Basin. The second manuscript reports on an empirical orthogonal function analysis of averaged sea surface height anomalies. The analysis is interpreted to describe the superposition of two principal circulation schemes. The former describes annual period, basin scale cyclonic circulation. The latter scheme reflects the southwestward propagation of $\sim$1.9 year period, gyre scale baroclinic long waves across the Aleutian Basin. The final paper reports on observations of topographic planetary waves associated with the Bering Slope Current. The model provides a possible resolution of some of the discrepancies between previously published circulation schemes for the Bering Sea.
• #### Carbon And Nitrogen Isotope Ratios In Bowhead Whales (Balaena Mysticetus) And Their Zooplankton Prey As Indicators Of Feeding Strategy And Environmental Change.

This study details regional, seasonal and inter-annual differences in $\delta\sp{13}$C and $\delta\sp{15}$N of zooplankton in the Bering and Chukchi Seas. These isotopic variations were correlated with inter-annual and long-term variations in $\delta\sp{13}$C and $\delta\sp{15}$N of bowhead whale baleen.<p> Statistical analyses indicate significant $\sp{13}$C-enrichment in Bering/Chukchi seas zooplankton relative to Beaufort Sea zooplankton, supporting the observed trends of $\sp{13}$C-depletion at higher latitudes and enrichment at lower latitudes. Bering/Chukchi seas zooplankton show a weak trend in $\sp{13}$C-depletion between 1987-1990, possibly due to inter-annual environmental changes affecting phytoplankton $\delta\sp{13}$C. The $\delta\sp{13}$C of baleen produced in fall/winter months is inversely correlated to sea surface temperature trends in the Bering Sea. Long-term changes in baleen $\delta\sp{13}$C may be helpful in assessing past climatic change.<p> The observed $\delta\sp{15}$N oscillations in baleen may be due to changes in trophic level of prey between seasonal feeding grounds or from physiological cycles in the bowhead whale. <p>
• #### Natural abundance of nitrogen(15) in a subarctic lake and biogeochemical implications to nitrogen cycling

Stable isotope ratios of nitrogen ($\delta\sp{15}$N) were employed to track the origin and fate of nitrogen in a subarctic lake, Alaska. The annual planktonic nitrogen cycle was dominated by N$\sb2$ fixation in spring and NH$\sb4\sp+$ assimilation in summer. In winter, microbial nitrification was the major sink for NH$\sb4\sp+$ and denitrification was accounted for most of the loss of NO$\sb3\sp-.$ The small isotope fractionation in nitrification is proposed as a result of substrate (NH$\sb4\sp+)$ limitation. The temporal and spatial homogeneity of the $\delta\sp{15}$N of dissolved organic nitrogen may be related to its large pool size and refractory nature. A stable isotope mass balance suggests that the winter phytoplankton was only composed of 10 to 20% of the suspended organic matter in water column due to low primary productivity during the ice cover period. A close correlation between $\delta\sp{15}$N of phytoplankton and $\delta\sp{15}$N of dissolved pools indicates that NH$\sb4\sp+$ was the predominant nitrogen source for non-N$\sb2$-fixing algae. The similarity of $\delta\sp{15}$N between a spring blue-green bloom and N$\sb2$ suggests an atmospheric origin for nitrogen. A mixing model estimated that the blue-green algal bloom derived approximately 70% of its nitrogen from molecular nitrogen. This fixed nitrogen was further transferred to higher trophic levels via the food chain and to other primary producers following mineralization. The $\delta\sp{15}$N of aquatic macrophytes indicates that non-rooted species obtained their nitrogen from the water column while rooted species obtained their nitrogen largely from the sediment. Evidence from dual isotope tracers ($\delta\sp{15}$N and $\delta\sp{13}$C) suggests that the zooplankton were supported by phytoplankton throughout the growing season despite an apparent abundance of detritus in the water column. Benthic fauna relied on either phytoplankton detritus or other organic matter in the sediment. The $\delta\sp{15}$N data exhibit only two to three trophic levels in both planktonic and the benthic communities in Smith Lake.
• #### Abundance, size, habitat utilization, and intrastream movement of juvenile coho salmon in a small southeast Alaskan stream

Aquatic habitat was measured, and juvenile coho salmon Oncorhynchus kisutch abundance and intrastream migrations were monitored in Kake Bake Creek, Alaska, between 1985 and 1986. Fry densities averaged 0.88, 0.33, and 0.11 fish/m2 during August, November, and March, respectively; parr densities averaged 0.15, 0.09, and 0.05 fish/m2, during August, November, and March, respectively. Fry were distributed evenly between riffle, glide, and pool habitat types during August, but not during November or March. Parr were distributed evenly in riffle and glide habitats during August, November and March. Stream areas containing pools and large woody debris tended to have higher coho densities; habitat was generally a significant predictor of juvenile abundance despite low R2 values. Fall immigrants totaled 1,434 coho, with 764 immigrating into beaver ponds. Fall immigrants were bright silver in color and several had sea-lice Calugus spp. attached near their anal fins. Between April 1 and June 2, 1986, 586 coho smolts emigrated from Kake Bake Creek; 172 had been fall immigrants.
• #### The ecology of a high-latitude rocky intertidal community: Processes driving population dynamics in Kachemak Bay, Alaska

The population dynamics and interactions of selected key species relative to community structure were investigated in the rocky intertidal of Kachemak Bay, southcentral Alaska (59$\sp\circ$35$\sp\prime$N, 151$\sp\circ$30$\sp\prime$W). The roles of recruitment processes and predation in regulating intertidal populations were emphasized in this investigation. Species cover was distinctly seasonal. Total cover typically exceeded 80% during the summer, especially in lower intertidal. Winter cover averaged 40-60%, with macroalgal cover varying up to six-fold between summer and winter. Barnacle recruitment varied both inter-annually and with respect to species. From 1991-1993, mean recruitment densities varied from 0.85-8.71 cm$\sp{-2}$ (range = 0-71 cm$\sp{-2}).$ In the upper intertidal, time-integrated summer recruit density of Semibalanus balanoides and Balanus glandula was 0.13 cm$\sp{-2}.$ Recruit density of S. cariosus in the low intertidal was 4.32 cm$\sp{-2}.$ In the low intertidal, recruits often saturated the surface, resulting in density-dependent mortality in two out of three years, a phenomenon which did not occur in the upper intertidal where space was never limiting. Predation was a significant source of mortality for barnacle recruits only in 1991, a poor recruitment year. However, predation by Nucella lima limited mussel (Mytilus trossulus) populations at some sites. Where N. lima density exceeded 100 m$\sp{-2},$ mussel cover was less than half that where Nucella was rare (31% vs. 72%). High densities of N. lima were estimated to remove 60-90% of mussels per season. Recruitment of the macroalga Fucus gardneri was almost 50 times greater in the presence of live barnacles than on bare rock surfaces or barnacle shells killed by heating. Recruitment in quadrats with tests of mechanically killed barnacles was intermediate. The results indicate that F. gardneri propagules are stimulated to attach by a chemical cue, probably a polypeptide, produced by barnacles. Based on population dynamics and species interactions investigated in Kachemak Bay, the mid- to low intertidal community appears to function similarly to the classical paradigm of regulation by competition and predation. The major exception is high inter-annual variability in predation relative to recruitment and competition.
• #### Numerical modeling study of the circulation of the Greenland Sea

This study is a simulation of the circulation of the Greenland Sea aimed at modeling some of the issues related to the Great Salinity Anomaly (GSA) and deep water formation using a primitive equation ocean general circulation model (Semtner, 1974). The features of the model include: (1) a high resolution, (2) real topography, (3) open boundaries at the south and north, and (4) temporally variable wind and thermohaline forcing. The model is used to study: (1) the spreading of a fresh water anomaly, (2) the mechanisms of cross frontal mixing that lead to deep water formation, (3) the general circulation of the deep and upper layers of the ocean and their dependence on wind and thermohaline forcing, and (4) the possible implications of meso-scale and large-scale variability on climate change. One of the major results of this work is the simulation of continental shelf waves propagating along the shelf slope of Greenland between 77$\sp\circ$N and 72$\sp\circ$N. Waves with a subinertial period of 17.2 hrs, a wavelength of 363 km, a phase speed of 586 cm/s and a group velocity of 409 cm/s, are found. Possible mechanism for generation of shelf waves is presented. It is suggested that some energy related with wave activity may support cross-frontal mixing in the East Greenland Current (EGC), where formation of the two main sources of North Atlantic Deep Water (e.g. Norwegian Sea Deep Water and Denmark Strait Overflow Water) have been reported. The results from the GSA simulation suggest that during the early stage of the GSA (e.g. during its propagation with the EGC to the south, in the late 1960s) when no observations are available, the fresh water signal is not being mixed into the interior circulation of the Greenland Sea gyre. The second experiment, representing recirculation of the GSA from the North Atlantic back into the Greenland Sea, in the late 1970s, shows freshening in the Greenland Sea gyre of comparable magnitudes ($-$0.05 to $-$0.1 psu) to the observed ones. These results agree with the earlier indirect measurements (Rhein, 1991; Schlosser et al., 1991) indicating dramatic reduction of deep water renewal in the Greenland Sea in the late 1970s and early 1980s. From the general circulation experiments it has been found that the ocean response to seasonal forcing is mainly barotropic. This implies a strong topographic control in the distribution of currents and hydrographic variables. Most of the areas of topographic steering which are simulated in the region have been reported in the literature. The so-called Molloy Deep eddy shows its direct dependence on the large scale dynamics affecting the northward flow of the West Spitsbergen Current (WSC), controlling this way a net mass transport into the Arctic Ocean. Simulations with different wind forcing suggest dependence of the Greenland Sea gyre circulation on the variations with time of the local wind forcing. Results indicate that monthly mean wind stress forcing probably underestimate wind forcing in the model. Analysis of surface, intermediate and deep ocean velocity fields compare reasonably well with observations.
• #### Decomposition and adsorption of peptides in Alaskan coastal marine sediments

In organic-rich coastal sediments, hydrolyzable amino acids make up a substantial fraction of the sedimentary content of organic nitrogen. How this organic nitrogen resists decomposition and is preserved in sediments is poorly understood. In order to investigate the factors controlling mineralization and preservation of hydrolyzable amino acids, decomposition and adsorption of peptides were studied in suboxic and anoxic pore water and sediments from Resurrection Bay (RB) and Skan Bay (SB), Alaska. Five tritium-labeled peptides, basic di-lysine, acidic di-glutamic acid, and neutral di-alanine, tri-alanine and hexa-alanine, were used as tracers. In filtered pore water, the hydrolysis rates were usually low. The exception was that the initial enzymatic hydrolysis of di-alanine and di-glutamic acid was rapid in SB pore water. The hydrolysis rates of both peptides increased with concentration. In sediments, hydrolysis was found to be the rate-limiting step of peptide decomposition. Alanyl and glutamyl peptides were hydrolyzed faster than lysyl peptide, and the hydrolysis rates among alanyl peptides decreased with increasing molecular weight. Peptide hydrolysis was affected more by molecular structure than by oxic or anoxic conditions. Adsorption of lysyl peptide to sediments was greater than that of other peptides. Basicity enhanced peptide adsorption more than increased molecular weight. Sedimentary organic matter was mainly responsible for peptide adsorption. The different patterns of peptide adsorption in RB and SB sediments were related to the greater total organic carbon concentration in SB sediment. Some of the peptide adsorption was irreversible. Adsorbed peptides were more resistant to biological decomposition than dissolved peptides. Adsorption may be an important step in the process of peptide preservation in sediments, and thus the preservation of sediment organic matter during early diagenesis.
• #### Threshold management strategies for exploited fish populations

Under a threshold management strategy, harvesting occurs at a constant rate but ceases when a population drops below a threshold. The threshold approach seeks to enhance long-term yield of a population and to maintain population renewability. I evaluated threshold management strategies for selected herring and pollock stocks in Alaska. First, I examined stock-recruitment data from 19 major herring stocks worldwide to provide the basis for evaluating threshold management strategies. Seventy-three percent of these stocks exhibited statistically significant density-dependence. Most stocks have compensatory, dome-shaped stock-recruitment curves. Then, I simulated threshold management strategies for eastern Bering Sea (EBS) pollock and herring and Prince William Sound (PWS) herring using a single-species model. I further examined seven alternative threshold estimation methods. Cohort analysis, catch-at-age analysis, and catch and population sampling yielded estimates of population parameters. The objective function was a weighted function of increased average yield and decreased standard deviation of yield over a planning horizon. Compared to a non-threshold approach, threshold management strategies increase the long-term average yields, stabilize population abundances, shorten rebuilding times, and increase management flexibility. For a maximum yield criterion and Ricker stock-recruitment models, optimal fishing mortalities are slightly above fishing mortalities at maximum sustained yield (MSY), and optimal threshold levels range from 40% to 60% of pristine biomass for EBS pollock, from 40% to 50% for EBS herring and from 30% to 60% for PWS herring. With fishing mortality at MSY and the criterion of equal trade-off between yield and its variation, optimal thresholds range from 20% to 30% of pristine biomass for pollock. With the status quo exploitation rate of 20%, optimal thresholds range from 10% to 25% of pristine biomass for EBS herring, and from 5% to 25% for PWS herring. Of the threshold estimation methods evaluated, default percentage of pristine biomass usually performs best. Loss of yield due to errors in threshold estimation is small, generally under 10%. A bout 15 to 20 years of data are required to obtain a reliable estimate of thresholds. With single-species dynamics, the form of the stock-recruitment curve, exploitation rate and management objective are the most important factors affecting optimal thresholds.