Food-Web Structure and Functioning of Coastal Marine Ecosystems: Alvarado Lagoon and Adjacent Continental Shelf, Northern Gulf of Mexico

Introduction:The aim of the present study was to develop a trophic model characterizing simultaneously the structure and function of the two coastal marine ecosystems: Alvarado Lagoon, Mexico and adjacent continental shelf, important area for penaeid shrimps and demersal fish species.Method:The model was based on the assumption of biomass balance and describes the trophic relationships, flows of energy, and transfer efficiency of the food web, and includes 66 functional groups.Results:Results shows that 33% of the aggregate biological community biomass comes mainly second and third trophic levels. Size of aggregate flows as well as the transfer effectiveness among groups, results like those saw in other similar works. Connectance index (CI) was 0.12, meaning that there is only 12% only of the total theoretical connections exist. Results also shows that primary producers and detritus (lagoon and shelf) contribute with 60.1% of the total ascendency. Our work can be the basis for future research, which allow us to contrast alternate hypotheses about the functioning of the system.

Can the Diving Industry Promote Marine Conservation and Enhance Environmental Awareness? (Sal Island, Cape Verde Case)

Introduction:The growth in tourism in the Cape Verdean Sal Island has been a cause as well as a consequence of a number of investments (both by the State and the private entities) to create synergies for that sector, ensuring and promoting the island’s touristic attractiveness. Such increases in the number of visitors, allied with the island’s marine biodiversity - that make up much of its economic potential - have caused the growth of the diving industry, which has become one of the biggest attractions for tourists. In light of the scarcity of scientific studies on the impacts associated with this activity and means for diver engagement, the local diving operators have proposed the development of several tools.Method:An Underwater Species Identification Guide and four underwater routes were proposed for four popular scuba diving sites off Santa Maria Bay (Sal Island, Cape Verde): “Kwarcit”, “Sargo”, “Três Grutas” and “Tchuklassa”. To better understand how the diving industry could promote environmental education, conservation and enhance biodiversity awareness among divers, we also established the diver tourists’ profile using an online survey.Results:A total of 347 respondents, of which 85% have higher education, showed that 67% stay more than seven nights in Cape Verde. Moreover, natural reefs are the 1stdive site to be visited, while artificial reefs preference increases after a second dive on the man-made reefs. The majority would recommend the use of the Underwater Species Identification Guide and the underwater routes.The results showed that divers strongly embrace the use of new tools for better understanding of diving site biodiversity and that the diving industry can play an important role in the enhancement of biodiversity awareness.

How to Kill Another Shark: A First Inside Look at a Potential Hunting Strategy of the Galapagos Shark

Background: Close-up videos of sharks foraging on other live sharks are rare, especially when it comes to footage of the entire duration of an event. Objective: Our goal was to present an in-depth analysis involving a Galapagos shark, Carcharhinus galapagensis, foraging on a whitetip reef shark, Triaenodon obesus. Method: A frame by frame video examination was used to highlight the different aspects of this bout. Results: Several behavioral characteristics, including the somewhat cryptic approach, the extensive shaking once the Galapagos shark had the whitetip reef shark between its jaws, and devouring tail-first, stand out in this event. Conclusion: The entire act appeared very smooth, indicating that this bout, or at least parts thereof, may represent a common hunting strategy for Galapagos sharks.

Food Web Structure Informs Potential Causes of Bimodal Size Structure in a Top Predator

Background: Assemblages of fishes in lakes and reservoirs in the western USA are dominated by non-native, large-bodied, piscivorous fishes that lack a shared evolutionary history. Top predators in these crowded systems are often characterized by unstable population dynamics and poor somatic growth rates. One such assemblage is in Fish Lake, located in southern Utah, USA, in which introduced lake trout (Salvelinus namaycush, Walbaum) exhibit a bimodal growth pattern. A few lake trout in Fish Lake grow rapidly to large size typical of the species; whereas, most never grow beyond 600 mm total length. Objective: To inform competitive interactions in this evolutionarily novel fish assemblage that might cause the low recruitment to large body size in lake trout, we characterized trophic niche (from stable isotope analysis of C and N) of all fishes in the lake. Methods: We used a Bayesian mixing model to describe the trophic niche and infer diet of lake trout and their potential prey, and we used Bayesian ellipse analysis to identify potential areas of high competition within the food web. Large lake trout feed mostly on small lake trout and splake (Salvelinus namaycush, Walbaum x Salvelinus fontinalis, Mitchill) despite availability of abundant yellow perch. (Perca flavescens, Mitchill). Small lake trout and splake feed mostly on zooplankton and exhibit substantial overlap of their trophic niche implying competition for food. Yellow perch and Utah chub (Gila atraria, Girard; formerly an important food item for lake trout in Fish Lake) exhibit extreme overlap of their trophic niche implying strong competitive interactions. Results: Our data suggest that lack of recruitment to large body size in lake trout may result from a reduction in availability of Utah chub resulting from competitive interactions with yellow perch, and increased competition from introduced splake for available prey. Conclusion: Management actions that may help ameliorate the poor somatic growth rates of most lake trout include efforts to reduce perch populations or increase vulnerability of perch to predation by lake trout, and removal of splake as a competitor of small lake trout.

Food Web Analysis of Jordanelle Reservoir, Utah, USA: Where Do All the Rainbows Go?

Background: Reservoir communities in the intermountain west are typically dominated by a mix of introduced fishes. Due to the non-coevolved interactions present in these communities, energy flow and trophic interactions may not facilitate optimal growth and survival for all species. It is difficult to predict how well each species will survive in such novel communities. One such community is in Jordanelle Reservoir in northern Utah, USA. Recently, low survival and recruitment of stocked rainbow trout (Onchorhynchus mykiss) have been observed in Jordanelle Reservoir. Objective: We characterize the food web structure of the fish community in Jordanelle reservoir to infer competitive or predatory interactions that might lead to a poor return of stocked rainbow trout. Methods: We performed a stable isotope analysis on the fish community in Jordanelle Reservoir and carried out niche space analyses using the software package Stable Isotope Bayesian Ellipse (SIBER) in R. Results: Small rainbow trout exhibit high competitive overlap with brown trout (Salmo trutta), smallmouth bass (Micropterus dolomieu), and yellow perch (Perca flavescens). In addition, large brown trout and large smallmouth bass may feed heavily on small rainbow trout. Conclusion: Food web analysis suggests that rainbow trout encounter a highly competitive and potentially high predation environment in Jordanelle reservoir, leading to observed low return rates.

Have Invasive Mysids (Mysis diluviana) Altered the Capacity of Osoyoos Lake, British Columbia to Produce Sockeye Salmon (Oncorhynchus nerka)?

Background:During 2005-13, at Osoyoos Lake, British Columbia, we investigated trophic relationships among fry ofOncorhynchus nerkaWalbaum (Sockeye and kokanee), a suite of limnetic planktivores includingMysis diluviana, and their zooplankton prey.Objectives:Our goal was to quantify the impacts that a recently introduced population ofMysiswould have on density, growth and survival of resident age-0 Sockeye Salmon.Methods:Evidence ofMysisimpact was based on (a) simple correlation analysis between various biophysical performance measures and (b) production and bioenergetics models used to identify the strength of bottom-up (i.e. production-driven) and top-down (i.e.consumption-driven) processes.Results:This nine-year study indicated that the Osoyoos Lake food web was strongly influenced by external events These included: large annual variations in river discharge, an earthen dam failure and effluent input from an Okanagan River tributary, and highly variable recruitment ofO. nerkafry given out-of-basin factors (harvest, marine survival) controlling adult salmon returns. Surprisingly, large annual variations inO. nerkarecruitment (0.63 - 7.0 million fry), did not induce significant “top-down” associations in growth, survival or subsequent production among the macro-planktivores (pelagic fish andMysis) and their zooplankton prey. A single significant correlation (p ≤ 0.05) betweenO. nerkafry abundance and their von BertalanffyW∞parameter emerged from a set of 14 potential top-down associations tested. By contrast, we identified several strongly positive “bottom-up” effects in which survival ofO. nerkafry was significantly associated (p ≤ 0.01) with annual variations in total zooplankton biomass,Daphniabiomass andEpischurabiomass. Our results indicate thatMysisplayed a dual role in the Osoyoos Lake pelagic food web. As predators, they accounted for an average (June-October) of 64% of the total prey biomass consumed by fish andMysis. As prey,Mysiscontributed an average of 35% of the prey biomass consumed by fish. Consumption by fish andMysistogether accounted for daily losses of only 4.5% of non-mysid zooplankton biomass and 34% of daily zooplankton production.Conclusion:We conclude that in all years, combined prey consumption byMysisand fish was never high enough, acting alone, to reduce the availability of their potential zooplankton forage base. However, we also estimate that in the absence ofMysis,O. nerkafry could experience a 43% increase in their daily food intake and that fish andMysismight control their principal prey taxa when exogenous factors (e.g.annual discharge) induced major reductions in zooplankton biomass. Finally, althoughMysishas clearly altered the energy flow pathways from plankton to fish in the Osoyoos Lake food-web, mysids have not precluded rebuildingO. nerkaabundance to levels at or exceeding historic maxima.

Landlocked Fall Chinook Salmon Maternal Liver and Egg Thiamine Levels in Relation to Reproductive Characteristics

Object: Landlocked fall Chinook Salmon Oncorhynchus tshawytscha in Lake Oahe, South Dakota, typically experience poor reproductive success. Introduction: Salmon diets consist of rainbow smelt Osmerus mordax and other potentially thiaminase-containing fish that could impact reproduction. Methods: The thiamine levels of spawning female Salmon, eggs, and reproductive characteristics, were measured in 2000, 2001, 2002, 2003, and 2005. Results: Thiamine concentrations varied significantly from year-to-year, with the highest mean values recorded in 2001 at 8.70 nmol/g in maternal livers and 28.80 nmol/g in eggs. Most of the thiamine in the eggs was present as free thiamine, while most of the thiamine in maternal livers was present as thiamine pyrophosphate. The lowest recorded egg total thiamine level was 2.75 nmol/g in 2000. Egg survival to hatch ranged from 20.7% in 2005 to 35.4% in 2002, and was not correlated to egg thiamine levels. Twenty-two spawns experienced total mortality prior to hatch, and had significantly lower egg free thiamine and total thiamine concentrations than eggs from the 77 successful spawns. The eggs from spawns with total mortality were also significantly smaller than those eggs from spawns that did survive, and were produced by females that weighed significantly less. Several small, but significant, correlations were observed between egg size and egg thiamine levels, and female size and liver thiamine. There was also a significant negative correlation between the number of eggs per spawning female and egg thiamine pyrophosphate, liver thiamine monophosphate, and liver total thiamine levels. Conclusion: In general, Lake Oahe Chinook Salmon eggs show little indication of thiamine deficiency in the years sampled, indicating other factors are likely responsible for poor egg survival.

When Do Mountain Whitefish (Prosopium williamsoni) Spawn? A Comparison of Estimates Based on Gonadosomatic Indices and Spawner and Egg Counts

Determining when fish spawn has major implications for effective fisheries management, particularly in dam-controlled rivers where reproductive potential may be affected by an altered hydrograph. Three methods for estimating spawn timing in riverine broadcast spawners were compared for their precision, effort and potential impact on a population of Mountain Whitefish in the regulated Lower Duncan River, Canada. The first method is based on the Gonadosomatic Index (GSI), which is a measure of the relative mass of an individual’s gonads. The second method is based on counts of aggregating adults, while the third method is based on passive egg collection using egg mats. Analysis of the GSI data provided the most precise estimates. It estimated that spawning occurred between October 30th and November 26th in 2010 and between November 8th and November 27th in 2011. Collection of GSI data required moderate effort and had some impact due to the need for lethal harvest. Analysis of the spawner counts using a simple Bayesian Area-Under-the-Curve model provided less precise estimates of spawn timing but the method likely had negligible impact on the population and required only moderate effort. Deployment of egg mats required high effort and collected insufficient information to derive statistical estimates of spawn timing. We discuss how information from different methods could be combined together into a single integrated model to maximize the precision while minimizing the effort and impact.

An Overview of the Current Status of Lake Naivasha Fishery: Challenges and Management Strategies

Lake Naivasha a shallow, freshwater body and a Ramsar site is found in the eastern arm of the Kenyan Rift Valley. This paper used published, unpublished and analyzed data to assess the status, challenges and management options for the fishery. Lake Naivasha fishery is based on exotic species that fluctuates depending on fishing regime, lake water level and aquatic plant concentrations. The fishery has been dominated by different species with the current catch contribution consisting mainly of common carp, Cyprinus carpio, Nile tilapia, Oreochromis niloticus, blue-spotted tilapia, O. leucostictus and African catfish, Clarias gariepinus. The minimum and maximum catch of 21 t yr-1 and 1181 t yr-1 was reported in 1997 and 2015, respectively. The main threats to the lake fishery are anthropogenic influences emanating from within the lake and its basin. The factors include intense fishing, exotic species introductions, water abstraction, lake level fluctuations, wetland utilization, eutrophication, and land degradation. There are also several conflicts of interest amongst the stakeholders in agriculture, fisheries, wildlife, tourism, conservation and geothermal electricity generation. There is fear that if the current trend persists, the lake and its fishery may be headed for extinction. The management measures instituted in the lake do not seem to have arrested reduction in fish catches nor reversed deterioration in water quality. For sustainable utilization of Lake Naivasha and its fishery, there is a need to consider a holistic ecosystem approach of the basin management. Additionally, all the relevant stakeholders should be involved in formulation and implementation of the decisions to manage the fishery.

A Simulation Study to Evaluate Survey Designs and Assessment Models for Estimation of Dungeness Crab (Cancer magister) Softshell Periods

Softshell Dungeness Crabs have inferior meat quality and are vulnerable to handling by harvesters; therefore, knowing when softshell periods occur is important for managing Dungeness Crab fisheries. A computer simulation was used to study the effectiveness of several survey designs and statistical models for estimating softshell periods which normally would be construed from crab shell condition data obtained from trap surveys. Survey designs varied in the number of years of data collection (1, 3, 5 or 10 years) and by the number and arrangement of sampling events per year. Three statistical models, including standardized catch-per-unit-effort (SCPUE), hierarchical, and generalized additive, were tested using catch-per-unit-effort data (CPUEs) or CPUE- transformed data. CPUEs were standardised by dividing CPUE estimates by the maximum CPUE obtained in the sample year, and then transformed using the complementary log-log function. In the hierarchical model, CPUEs were modelled using a lognormal distribution, assuming the expected logarithms of CPUEs are a quadratic function of days plus a random normal error. CPUE-transformed data were modelled using a normal distribution, assuming expected values are a quadratic function of days in the SCPUE model or a spline smooth function of days in the generalized additive model. Results suggest the best survey design requires a relatively high number (6 or 11) of sampling events during several key consecutive months which contain the softshell period, and fewer sampling events during those months when softshell crab abundance is low. A minimum 3 years of data collection is required to produce reliable outputs. The hierarchical model performs best, slightly better than the SCPUE model. Use of the generalized additive model is not recommended.