Doran M. Mason

Compartments revealed in food-web structure

Compartments in food webs are subgroups of taxa in which many strong interactions occur within the subgroups and few weak interactions occur between the subgroups. Theoretically, compartments increase the stability in networks, such as food webs. Compartments have been difficult to detect in empirical food webs because of incompatible approaches or insufficient methodological rigour. Here we show that a method for detecting compartments from the social networking science identified significant compartments in three of five complex, empirical food webs. Detection of compartments was influenced by food web resolution, such as interactions with weights. Because the method identifies compartmental boundaries in which interactions are concentrated, it is compatible with the definition of compartments. The method is rigorous because it maximizes an explicit function, identifies the number of non-overlapping compartments, assigns membership to compartments, and tests the statistical significance of the results. A graphical presentation reveals systemic relationships and taxa-specific positions as structured by compartments. From this graphic, we explore two scenarios of disturbance to develop a hypothesis for testing how compartmentalized interactions increase stability in food webs.

Spatially-explicit Models of Fish Growth Rate

Abstract Local density-dependent processes may significantly affect production dynamics at the population and ecosystem level. These processes are often nonlinear and occur at rates strongly influenced by the physical habitat. The spatial patchiness of both fish density and habitat has hampered our ability to separate the effects of density and habitat on production. In this paper, we introduce the concept of fish growth rate potential. Growth rate potential integrates physiologically-based models of fish growth rate with high-resolution spatial data on prey sizes, prey density, and the physical environment. By combining the strengths of bioenergetics models to simulate fish growth and of bioacoustics to measure fish density and size in a spatially-explicit framework, the approach overcomes some of the difficulties inherent in a spatially-patchy environment. The result is a two-dimensional, nonlinear model of fish growth and system production. Predator behavior and foraging algorithms provide additional m...

Predation by alewives on larvae of yellow perch in Lake Ontario

Abstract Alewives Alosa pseudoharengus preyed upon larval yellow perch Perca flavescens during late April and early May in a Lake Ontario embayment. Based on stomach analyses of 1,308 alewives collected during the springs of 1984–1986, predation on larval yellow perch occurred primarily at night in littoral areas during the 2 weeks immediately following peak hatch of yellow perch. Among stomachs of all alewives caught at night in littoral areas, 30% in 1985 (N = 43) and 41% in 1986 (N = 71) contained yellow perch larvae. Mean numbers of whole larvae per alewife stomach were 3.8 in 1985 and 32.6 in 1986. As many as 361 intact larval yellow perch were found in one stomach. Mean total lengths of yellow perch consumed on two dates in mid-May 1986 were 7.3 and 7.6 mm, compared to mean sizes of 8.0 and 8.6 mm of larvae living within the bay. We suggest predation by alewives on yellow perch larvae may reduce yellow perch recruitment in Lake Ontario as well as in other Great Lakes.

Recruitment Variability of Alewives in Lake Michigan

We used a long-term series of observations on alewife Alosa pseudoharengusabun- dance that was based on fall bottom-trawl catches to assess the importance of various abiotic and biotic factors on alewife recruitment in Lake Michigan during 1962-2002. We first fit a basic Ricker spawner-recruit model to the lakewide biomass estimates of age-3 recruits and the cor- responding spawning stock size; we then fit models for all possible combinations of the following four external variables added to the basic model: an index of salmonine predation on an alewife year-class, an index for the spring-summer water temperatures experienced by alewives during their first year in the lake, an index of the severity of the first winter experienced by alewives in the lake, and an index of lake productivity during an alewife year-classs second year in the lake. Based on an information criterion, the best model for alewife recruitment included indices of salmonine predation and spring-summer water temperatures as external variables. Our analysis corroborated the contention that a decline in alewife abundance during the 1970s and early 1980s in Lake Michigan was driven by salmonine predation. Furthermore, our findings indicated that the extraordinarily warm water temperatures during the spring and summer of 1998 probably led to a moderately high recruitment of age-3 alewives in 2001, despite abundant salmonines. A key problem in fisheries research is predicting recruitment from a given level of spawning stock size (Sissenwine et al. 1988; Myers et al. 2001; Kehler et al. 2002). Fish recruitment can be strong- ly influenced by many abiotic and biotic factors, including water temperature, water movements, predation, and spawning stock size (Sissenwine 1984; Hilborn and Walters 1992). Although im- portant factors affecting recruitment may vary across ecosystems (Madenjian et al. 1996), inter- esting patterns may emerge by comparing recruit- ment analyses for populations of a species across ecosystems (Myers 1998). An invasion of alewives Alosa pseudoharengus during the 1940s proved to be an important stressor to the Lake Michigan ecosystem (Wells and

Biophysical Model of Larval Yellow Perch Advection and Settlement in Lake Michigan

ABSTRACT Potential for large-scale physical transport processes to affect recruitment of Lake Michigan yellow perch (Perca flavescens) was studied by examining the variation in larval distribution, growth rate, and settlement during June–August 1998–2003 using a 3D particle transport model linked with an individual-based bioenergetics growth model. In all years, virtual larvae were released nearshore in southwestern Lake Michigan, a known and important spawning region for yellow perch. For any given year, the same circulation pattern and water temperature either promoted or reduced yellow perch settlement depending on the consumption rates and settlement size chosen in the growth model. Increased consumption increased the number of settled larvae and expanded the total area where larvae settled, whereas increased settlement size reduced the number of settled larvae and reduced the overall settlement area. Interannual variability in circulation patterns and water temperature also resulted in contrasting larval settlement rates, settlement locations, and size of settlement areas between years. Model predictions were most consistent with field observations of age-0 yellow perch from Illinois and Michigan waters when settlement was assumed to occur at 50 mm. Moreover, our model suggests that larvae originating from southwestern Lake Michigan can recruit anywhere within the southern basin and even in the northern basin. Future model improvement will require information on the relative contribution of various sectors to the larval pool, their distribution with reference to the hydrodynamic landscape, the feeding and growth of yellow perch during their pelagic phase, and the size at transition to demersal stage.

Density-dependent habitat selection and performance by a large mobile reef fish.

Many exploited reef fish are vulnerable to overfishing because they concentrate over hard-bottom patchy habitats. How mobile reef fish use patchy habitat, and the potential consequences on demographic parameters, must be known for spatially explicit population dynamics modeling, for discriminating essential fish habitat (EFH), and for effectively planning conservation measures (e.g., marine protected areas, stock enhancement, and artificial reefs). Gag, Mycteroperca microlepis, is an ecologically and economically important warm-temperate grouper in the southeastern United States, with behavioral and life history traits conducive to large-scale field experiments. The Suwannee Regional Reef System (SRRS) was built of standard habitat units (SHUs) in 1991-1993 to manipulate and control habitat patchiness and intrinsic habitat quality, and thereby test predictions from habitat selection theory. Colonization of the SRRS by gag over the first six years showed significant interactions of SHU size, spacing, and reef age; with trajectories modeled using a quadratic function for closely spaced SHUs (25 m) and a linear model for widely spaced SHUs (225 m), with larger SHUs (16 standardized cubes) accumulating significantly more gag faster than smaller 4-cube SHUs (mean = 72.5 gag/16-cube SHU at 225-m spacing by year 6, compared to 24.2 gag/4-cube SHU for same spacing and reef age). Residency times (mean = 9.8 mo), indicative of choice and measured by ultrasonic telemetry (1995-1998), showed significant interaction of SHU size and spacing consistent with colonization trajectories. Average relative weight (W(r)) and incremental growth were greater on smaller than larger SHUs (mean W(r) = 104.2 vs. 97.7; incremental growth differed by 15%), contrary to patterns of abundance and residency. Experimental manipulation of shelter on a subset of SRRS sites (2000-2001) confirmed our hypothesis that shelter limits local densities of gag, which, in turn, regulates their growth and condition. Density-dependent habitat selection for shelter and individual growth dynamics were therefore interdependent ecological processes that help to explain how patchy reef habitat sustains gag production. Moreover, gag selected shelter at the expense of maximizing their growth. Thus, mobile reef fishes could experience density-dependent effects on growth, survival, and/or reproduction (i.e., demographic parameters) despite reduced stock sizes as a consequence of fishing.

Classifying and Forecasting Coastal Upwellings in Lake Michigan Using Satellite Derived Temperature Images and Buoy Data

ABSTRACT Coastal upwellings are common in the Great Lakes but have lacked enumeration and systematic classification of spatial extent, frequency, duration, and magnitude. Near real-time sea surface temperature (SST) images derived from the Advanced Very High Resolution Radiometer (AVHRR) provide indices of upwelling events, but visual inspection of daily images can be tedious. Moreover, the definition of what constitutes an upwelling from AVHRR data is subjective. We developed a semi-automated method to classify upwellings during the period of thermal stratification using daily, cloud-free surface temperature charts from AVHRR SST data. Then we statistically evaluated the location, frequency, magnitude, extent, and duration of upwelling events in Lake Michigan from 1992–2000. Further, we analyzed meteorological data from the National Data Buoy Center buoys in an attempt to improve the reliability of the classification and to provide a means for future forecast of coastal upwelling. Although variable, upwelling events along the western shoreline were preceded by 4 days of southerly and west-to-north-westerly winds, while upwelling events occurring along the eastern shore were preceded by 4 days of northerly winds. Probability of an upwelling event occurring was a function of the direction-weighted wind speed, reaching a 100% probability at direction weighted wind speeds of 11 m s−1 for the western shore. Probability of an upwelling occurrence along the east coast reached 73% at 11 m s−1 and 100% at 13 m s−1. Continuous measurements of wind data with a sufficient temporal resolution are required during the entire upwelling season to improve the predictability of upwellings.

Hatch Dates, Growth, Survival, and Overwinter Mortality of Age-0 Alewives in Lake Michigan: Implications for Habitat-Specific Recruitment Success

Abstract Alewives Alosa pseudoharengus are key components of Laurentian Great Lakes ecosystems and spawn in multiple habitat types. Exploration of alewife early life history dynamics within these different habitats should help identify important recruitment processes. During 2001-2003, we quantified physical (temperature, transparency) and biotic (chlorophyll a, zooplankton densities) habitat factors and collected age-0 alewives (using ichthyoplankton nets and trawls) in a nearshore region of Lake Michigan and Muskegon Lake, Michigan (a drowned river mouth lake connected to Lake Michigan). We characterized alewife hatch dates, individual condition, growth, mortality, and size-dependent overwinter survival to infer differences in habitat-specific recruitment success. Temperature, turbidity, chlorophyll-a concentrations, and densities of zooplankton prey were consistently higher in Muskegon Lake than in nearshore Lake Michigan. On average, young alewives in Muskegon Lake hatched earlier, grew faster, were i...

Hydroacoustic estimates of abundance and spatial distribution of pelagic prey fishes in Western Lake Superior

Lake herring (Coregonus artedi) and rainbow smelt (Osmerus mordax) are a valuable prey resource for the recovering lake trout (Salvelinus namaycush) in Lake Superior. However, prey biomass may be insufficient to support the current predator demand. In August 1997, we assessed the abundance and spatial distribution of pelagic coregonines and rainbow smelt in western Lake Superior by combining a 120 kHz split beam acoustics system with midwater trawls. Coregonines comprised the majority of the midwater trawl catches and the length distributions for trawl caught fish coincided with estimated sizes of acoustic targets. Overall mean pelagic prey fish biomass was 15.56 kg ha−1 with the greatest fish biomass occurring in the Apostle Islands region (27.98 kg ha−1), followed by the Duluth Minnesota region (20.22 kg ha−1), and with the lowest biomass occurring in the open waters of western Lake Superior (9.46 kg ha−1). Biomass estimates from hydroacoustics were typically 2–134 times greater than estimates derived from spring bottom trawl surveys. Prey fish biomass for Lake Superior is about order of magnitude less than acoustic estimates for Lakes Michigan and Ontario. Discrepancies observed between bioenergetics-based estimates of predator consumption of coregonines and earlier coregonine biomass estimates may be accounted for by our hydroacoustic estimates.

Effects of spatial scale and foraging efficiency on the predictions made by spatially-explicit models of fish growth rate potential

SynopsisSpatially-explicit modeling of fish growth rate potential is a relatively new approach that uses physical and biological properties of aquatic habitats to map spatial patterns of fish growth rate potential. Recent applications of spatially-explicit models have used an arbitrary spatial scale and have assumed a fixed foraging efficiency. We evaluated the effects of spatial scale, predator foraging efficiency (combined probabilities of prey recognition, attack, capture, and ingestion), and predator spatial distribution on estimates of mean growth rate potential of chinook salmon,Oncorhynchus tshawytscha. We used actual data on prey densities and water temperatures taken from Lake Ontario during the summer, as well as, simulated data assuming binomial distribution of prey. Results show that a predator can compensate for low foraging efficiency by inhabiting the most profitable environments (regions of high growth rate potential). Differences exist in predictions of growth rate potential across spatial scales of observation and a single scale may not be adequate for interpreting model results across seasons. Continued refinements of this modeling approach must focus on the assumptions of stationary distributions of predator and prey populations and predator foraging tactics.