Environmental change drives long‐term recruitment and growth variation in an estuarine fish

How individuals respond to environmental change determines the strength and direction of biological processes like recruitment and growth that underpin population productivity. Ascertaining the relative importance of environmental factors can, however, be difficult given the numerous mechanisms through which they affect individuals. This is especially true in dynamic and complex estuarine environments. Here, we develop long‐term otolith‐based indices of recruitment and growth for estuary perch Percalates colonorum (Bemm River, Australia), to explore the importance of intrinsic (individual, demographic) and extrinsic (hydrologic, climatic, density‐dependent) factors in driving estuarine fish productivity. Analyses involved a novel zero‐inflated specification of catch curve regression and mixed effects modelling. The 39 years of recruitment and 46 years of growth data, spanning a period of environmental change including severe drought, displayed considerable inter‐annual variation. Recruitment success was strongly related to high freshwater inflows during the spawning season, suggesting that these conditions act as spawning cues for adults and potentially provide favourable conditions for larvae. Individuals displayed age‐dependent growth, with highest rates observed at younger ages in years characterized by warm temperatures, and to a lesser degree, greater magnitude base inflow conditions. We detected systematic among‐year‐class growth differences, but these were not attributable to year class strength, suggesting that environmental conditions experienced by individuals as juveniles can have long‐lasting effects of greater importance to population productivity than density‐dependent growth responses. The primacy of temperature in driving growth variation highlights that under‐appreciated climatic variation can affect estuarine fish productivity through direct physiological and indirect food web mechanisms. We predict that climatic warming will promote individual growth in southerly populations of P. colonorum but concurrently limit recruitment due to forecast reductions in spawning season river discharge. Disparate trait responses are likely in other fishes as they respond to multiple and changing environmental drivers, making predictions of future population productivity challenging.     

The nature of fisheries- and farming-induced evolution

Humans have a penchant for unintentionally selecting against that which they desire most. In fishes, unprecedented reductions in abundance have been associated with unprecedented changes in harvesting and aquaculture technologies. Fishing, the predominant cause of fish-population collapses, is increasingly believed to generate evolutionary changes to characters of import to individual fitness, population persistence and levels of sustainable yield. Human-induced genetic change to wild populations can also result from interactions with their domesticated counterparts. Our examination of fisheries- and farming-induced evolution includes factors that may influence the magnitude, rate and reversibility of genetic responses, the potential for shifts in reaction norms and reduced plasticity, loss of genetic variability, outbreeding depression and their demographic consequences to wild fishes. We also suggest management initiatives to mitigate the effects of fisheries- and farming-induced evolution. Ultimately, the question of whether fishing or fish farming can cause evolutionary change is moot. The key issue is whether such change is likely to have negative conservation- or socio-economic consequences. Although the study of human-induced evolution on fishes should continue to include estimates of the magnitude and rate of selection, there is a critical need for research that addresses short- and long-term demographic consequences to population persistence, plasticity, recovery and productivity.     

Issues of ecosystem-based management of forage fisheries in “open” non-stationary ecosystems: the example of the sardine fishery in the Gulf of California

The Gulf of California system presents major challenges to the still developing frameworks for ecosystem-based management (EBM). It is very much an open system and is intermittently subject to important influxes of migratory visitors, including large pelagic predatory fishes and small pelagic forage fishes. These migrants include the more tropical species from the coastal ecosystems to the south and perhaps subtropical sardines and anchovies from the California Current upwelling system. In addition to the multi-annual ENSO-scale and what may seem to be rather erratic episodes of major population incursions, the Gulf presents nonstationary, transient aspects on a variety of longer time scales. Moreover, the removal of top predators by commercial and sport fisheries has introduced trends that must be affecting the entire ecosystem, and certainly the forage fishes that are their major prey base. In addition to size limits, fishing seasons, area closures and license limitations, the fishery is managed by an ad hoc adaptive management system, in which the fishing season can be shortened or additional areas closed to fishing if pre-season exploratory fishing surveys indicate a shortage of small pelagic fishes on the fishing grounds. Whether this system is likely to be sustainable in the long term is difficult to determine, given the potential for rapid changes in the system because of environmental changes and/or feedbacks within the food web. Thus it appears that innovative management frameworks, among other things utilizing the comparative method, may be required in order to determine defensible tradeoffs between precaution and resource utilization.     

Strong direct and inconsistent indirect effects of fishing found using stereo-video: Testing indicators from fisheries closures

Candidate indicators of the direct and indirect effects of fishing can be developed by investigating fisheries closures. We tested a suite of such indicators in areas open to fishing but with suspected differences in effort, using baited remote underwater stereo-video methods. In particular, we predicted that greater fishing would result in decreased biomass of high risk target species and indirectly increase the biomass of small-bodied non-target species. As predicted, the biomass of target species was found to be greater in areas of lower fishing effort and in deeper waters. However, no indirect effects of fishing were detected and any community-level effects were driven by differences in the biomass of target species. In particular, assemblage length class analysis of covariance (ANCOVA), size spectra analysis and the abundance-biomass comparison (ABC) method did not provide any evidence of indirect effects of fishing. The magnitude of the differences in fishing effort between the two areas sampled, may be sufficient to significantly affect target fisheries species, but insufficient to lead to indirect effects on non-target populations. It is also possible that the predicted indirect effects do not occur in this assemblage, due to weak trophic linkages between species. Differences observed using the ABC method were attributed to variation in the abundance of large herbivorous fishes, which are not fished. We also found assemblage length class ANCOVA and size spectra to be insensitive to the direct effects of fishing where large numbers of non-target individuals are sampled along with fished species. We suggest diet studies and comparisons across stronger gradients in fishing pressure to further investigate the indirect effects of fishing in this assemblage.     

Ecological effects of longline fishing and climate change on the pelagic ecosystem off eastern Australia

Pelagic longline fisheries target (or catch incidently) large apex predators in the open ocean (e.g. tunas, billfish and sharks) and have the potential to disrupt the ecosystem functionality if these predators exert strong top–down control. In contrast, warming of oceans from climate change may increase bottom–up effects from increases in primary productivity. An ecosystem model of a large pelagic ecosystem off eastern Australia was constructed to explore the potential ecological effects of climate change and longlining by Australia’s Eastern Tuna and Billfish Fishery. The model reproduced historic biomass and fishery catch trends from 1952 to 2006 for seven functional groups. Simulated changes in fishing effort and fishing mortality rate on individual target species from 2008 to 2018 resulted in only modest (<20%) changes in the biomass of target species and their direct predators or competitors. A simulated increase in phytoplankton biomass due to climate change resulted in only small increases (<11%) in the biomass of all groups. However, climate-related changes to the biomass of micronekton fish (−20%) and cephalopods (+50%) resulted in trophic cascades. Our results suggest there may be ecological redundancy among high trophic level predators since they share a diverse suite of prey and collectively only represent <1% of the total system biomass. In contrast, micronekton fishes and cephalopods have high biomasses and high production and consumption rates and are important as both prey and predators. They appear to exert ‘wasp–waist’ control of the ecosystem rather than top–down or bottom–up processes reported to drive other pelagic systems.     

Empirical equations for estimating maximum length from length at first maturity

Maximum length reached by fishes is an important parameter that is highly correlated with metabolism and most other life-history traits. However, obtaining maximum length estimates for commercial fishes has become difficult due to the extirpation of large specimens by intensive fishing. Empirical equations are presented that can be used to derive maximum length of fish from length at first maturity, and vice versa.     

Evolutionary response to size-selective mortality in an exploited fish population

Many collapsed fish populations have failed to recover after a decade or more with little fishing. This may reflect evolutionary change in response to the highly selective mortality imposed by fisheries. Recent experimental work has demonstrated a rapid genetic change in growth rate in response to size-selective harvesting of laboratory fish populations. Here, we use a 30-year time-series of back-calculated lengths-at-age to test for a genetic response to size-selective mortality in the wild in a heavily exploited population of Atlantic cod (Gadus morhua). Controlling for the effects of density- and temperature-dependent growth, the change in mean length of 4-year-old cod between offspring and their parental cohorts was positively correlated with the estimated selection differential experienced by the parental cohorts between this age and spawning. This result supports the hypothesis that there have been genetic changes in growth in this population in response to size-selective fishing. Such changes may account for the continued small size-at-age in this population despite good conditions for growth and little fishing for over a decade. This study highlights the need for management regimes that take into account the evolutionary consequences of fishing.     

Global assessment of the status of coral reef herbivorous fishes: evidence for fishing effects

On coral reefs, herbivorous fishes consume benthic primary producers and regulate competition between fleshy algae and reef-building corals. Many of these species are also important fishery targets, yet little is known about their global status. Using a large-scale synthesis of peer-reviewed and unpublished data, we examine variability in abundance and biomass of herbivorous reef fishes and explore evidence for fishing impacts globally and within regions. We show that biomass is more than twice as high in locations not accessible to fisheries relative to fisheries-accessible locations. Although there are large biogeographic differences in total biomass, the effects of fishing are consistent in nearly all regions. We also show that exposure to fishing alters the structure of the herbivore community by disproportionately reducing biomass of large-bodied functional groups (scraper/excavators, browsers, grazer/detritivores), while increasing biomass and abundance of territorial algal-farming damselfishes (Pomacentridae). The browser functional group that consumes macroalgae and can help to prevent coral–macroalgal phase shifts appears to be most susceptible to fishing. This fishing down the herbivore guild probably alters the effectiveness of these fishes in regulating algal abundance on reefs. Finally, data from remote and unfished locations provide important baselines for setting management and conservation targets for this important group of fishes.     

Fecundity and the survival conditions of the offspring of the Baltic Cod <Emphasis Type="Italic">Gadus morhua callarias</Emphasis> (Gadidae)

The interannual and ontogenetic dynamics of the fecundity of the Baltic cod Gadus morhua callarias and the dependence of the number of the offspring on the reproduction conditions were studied. The absolute individual fecundity and the relative individual fecundity had no pronounced interannual dynamics for the period of 1994?2013. The maximal population fecundity was observed in early 1980, the minimal, in 1990?2000. Nearly every year, the highest contribution to the population fecundity belongs to the 4?5-year old females. The ranking of the survival rates and the year-class strength for the females at the age of 2 years showed that the emergence of the generations of high and average productivity (1980?1986) referred to the unfavorable and moderate survival conditions during the early ontogenesis. During the period of small stock (after 1987), the conditions were mostly favorable and moderate. It is suggested that the significant role in the offspring dynamics belongs to the factors linked to the population density. After 2005, favorable survival conditions promoted the increase of the Baltic cod abundance; this increase was also supported by the decrease of the commercial fishing and by the sporadic large-scale advective processes.     

Scale-Up of flat plate photobioreactors considering diffuse and direct light characteristics

This study investigates the scaling of photobioreactor productivity based on the growth of Nannochloropsis salina incorporating the effects of direct and diffuse light. The scaling and optimization of photobioreactor geometry was analyzed by determining the growth response of a small-scale system designed to represent a core sample of a large-scale photobioreactor. The small-scale test apparatus was operated at a variety of light intensities on a batch time scale to generate a photosynthetic irradiance (PI) growth dataset, ultimately used to inform a PI growth model. The validation of the scalability of the PI growth model to predict productivity in large-scale systems was done by comparison with experimental growth data collected from two geometrically different large-scale photobioreactors operated at a variety of light intensities. For direct comparison, the small-scale and large-scale experimental systems presented were operated similarly and in such a way to incorporate cultivation relevant time scales, light intensities, mixing, and nutrient loads. Validation of the scalability of the PI growth model enables the critical evaluation of different photobioreactor geometries and design optimization incorporating growth effects from diffuse and direct light. Discussion focuses on the application of the PI growth model to assess the effect of diffuse light growth compared to direct light growth for the evaluation of photobioreactors followed by the use of the model for photobioreactor geometry optimization on the metric of areal productivity.     

Using rapid assessment and demographic methods to evaluate the effects of fishing on Heterodontus portusjacksoni off far-eastern Victoria, Australia

A rapid semi-quantitative ecological risk assessment method (productivity and susceptibility analysis) indicated that, despite its low biological productivity, the Port Jackson shark Heterodontus portusjacksoni is at low risk to all fishing methods in far-eastern Victoria, Australia, under the present fishing practices, because of its low catch susceptibility. The risk to this population, however, would increase if the shark gillnet fishery operating in the region were to retain the species as a by-product. Demographic analysis indicated that the species has medium intrinsic population growth rate and potential rebound in comparison with other chondrichthyan species, juveniles have higher elasticity than mature females and both juvenile and mature females have higher elasticities than hatchlings. Because of its low biological productivity and moderate resilience to the effects of fishing, cautious management measures will be necessary to ensure the sustainable use of H. portusjacksoni if its marketing increases in the future. Information on the dynamics of a population that is valuable to provide management advice can be obtained through demographic methods, but rapid assessment methods can also provide complementary information on the effects of fishing by considering the catch susceptibility of the population to each fishing method.     

Effects of Spearfishing on Reef Fish Populations in a Multi-Use Conservation Area

Although spearfishing is a popular method of capturing fish, its ecological effects on fish populations are poorly understood, which makes it difficult to assess the legitimacy and desirability of spearfishing in multi-use marine reserves. Recent management changes within the Great Barrier Reef Marine Park (GBRMP) fortuitously created a unique scenario by which to quantify the effects of spearfishing on fish populations. As such, we employed underwater visual surveys and a before-after-control-impact experimental design to investigate the effects of spearfishing on the density and size structure of target and non-target fishes in a multi-use conservation park zone (CPZ) within the GBRMP. Three years after spearfishing was first allowed in the CPZ, there was a 54% reduction in density and a 27% reduction in mean size of coral trout (Plectropomus spp.), the primary target species. These changes were attributed to spearfishing because benthic habitat characteristics and the density of non-target fishes were stable through time, and the density and mean size of coral trout in a nearby control zone (where spearfishing was prohibited) remained unchanged. We conclude that spearfishing, like other forms of fishing, can have rapid and substantial negative effects on target fish populations. Careful management of spearfishing is therefore needed to ensure that conservation obligations are achieved and that fishery resources are harvested sustainably. This is particularly important both for the GBRMP, due to its extraordinarily high conservation value and world heritage status, and for tropical island nations where people depend on spearfishing for food and income. To minimize the effects of spearfishing on target species and to enhance protection of functionally important fishes (herbivores), we recommend that fishery managers adjust output controls such as size- and catch-limits, rather than prohibit spearfishing altogether. This will preserve the cultural and social importance of spearfishing in coastal communities where it is practised.     

Genetic variation in life-history reaction norms in a marine fish

Neither the scale of adaptive variation nor the genetic basis for differential population responses to the environment is known for broadcast-spawning marine fishes. Using a common-garden experimental protocol, we document how larval growth, survival and their norms of reaction differ genetically among four populations of Atlantic cod (Gadus morhua). These traits, and their plastic responses to food and temperature, differed across spatial scales at which microsatellite DNA failed to detect population structure. Divergent survival reaction norms indicate that warm-water populations are more sensitive to changes in food, whereas cold-water populations are more sensitive to changes in temperature. Our results suggest that neither the direction nor the magnitude of demographic responses to environmental change need be the same among populations. Adaptive phenotypic plasticity, previously undocumented in marine fishes, can significantly influence the probability of recovery and persistence of collapsed populations by affecting their ability to respond to natural and anthropogenic environmental change.     

Investigation of age composition and growth of cod <Emphasis Type="Italic">Gadus morhua morhua</Emphasis> of the Barents Sea in connection with the estimation of its stocks state

The age composition of catches and growth of the cod Gadus morhua morhua in the Barents Sea were investigated on the materials, collected in 2002–2006 in comparison with the data for the previous fishing periods. Analysis of data on the age composition indicated that the range of age groups had not changed since the beginning of the past century; however, a part of the younger age groups in the catches increased. The likeness of the cod sizes in the present period with those of the individuals of the same age caught in 1949–1980 and the significant decrease of the sizes as compared with those observed in 1981–1993 are observed. It is possible to suppose that the present decrease of indices of cod growth is connected with an increase of its numbers. A selective effect of the trawl fishery withdrawing in the first place the quick-growing individuals can be another factor that determined the decrease of body sizes of the fishes of the same age.     

Effects of the large-scale uncontrolled fertilisation process along the continental coastal North Sea

In this paper, effects of eutrophication in selected compartments of the North Sea ecosystem are discussed, encompassing the possibly positive effects of nutrient enrichment. Based on a variety of studies, impacts on biomass of phytoplankton, macrozoobenthos, microzooplankton, shrimps and fishes and productivity are presented. Enhanced nutrient concentrations and loadings can be observed in several coastal areas of the North Sea. As a result, increases in the concentration, production and changes in the species composition was observed in the phytoplankton. In addition, there are some indications for an increased biomass of macrozoobenthos, whereas an increase in microzooplankton can only be assumed from mesocosm experiments. A concomitant increase of higher trophic levels such as shrimps and fishes, as observed in some coastal regions of the North Sea, is difficult to link directly to eutrophication due to a lack of conclusive field observations showing the causality of the changes. That the large fertilisation process in the North Sea has led to a series of changes is, however, without doubt. The answer, to what extent these can be claimed as being harmless, positive or negative from the anthropogenic point of view, is hampered by the lack of good assessment criteria for marine ecosystems and requires a thorough analysis of all compartments involved by means of long-term-series long enough to discriminate between man-made and natural variability.     

A rough guide to population change in exploited fish stocks

Interpreting how populations will change in response to exploitation is essential to the sound management of fish stocks. While deterministic models can be of use in evaluating sustainable fishing rates, the inherent variability of fish populations limits their value. In this paper a probabilistic approach is investigated which avoids having to make strong assumptions about the functional relationship between spawning stock size and the annual number of young fish (recruits) produced. Empirical probability distributions for recruits are derived, conditioned on stock size, and used to indicate likely stock changes under different fishing mortality rates. The method is applied to cod ( Gadus morhua ) in the North Sea to illustrate how population change can be inferred and used by fishery managers to choose fishing mortality rates which are likely to achieve sustainable exploitation.     

Loss of trophic complexity in saline prairie lakes as indicated by stable-isotope based community-metrics

Variations in climate, watershed characteristics and lake-internal processes often result in a large variability of food-web complexity in lake ecosystems. Some of the largest ranges in these environmental parameters can be found in lakes across the northern Great Plains as they are characterized by extreme gradients in respect to lake morphometry and water chemistry, with individual parameters often varying over several orders of magnitude. To evaluate the effects of environmental conditions on trophic complexity in prairie lake food-webs, we analyzed carbon and nitrogen stable isotopes of fishes, zooplankton and littoral macroinvertebrates in 20 lakes across southern Saskatchewan. Our two-year study identified very diverse patterns of trophic complexity, with was predominantly associated with among-lake differences. Small but significant temporal effects were also detected, which were predominantly associated with changes in productivity. The most influential parameters related to changes in trophic complexity among lakes were salinity, complexity of fish assemblage, and indicators of productivity (e.g. nutrients, Chl a). Generally, trophic diversity, number of trophic levels, and trophic redundancy were highest in productive freshwater lakes with diverse fish communities. Surprisingly, mesosaline lakes that were characterized by very low or no predation pressure from fishes were not colonized by invertebrate predators as it is often the case in boreal systems; instead, trophic complexity was further reduced. Together, prairie lake food-webs appear to be highly sensitive to changes in salinity and the loss of piscivorous fishes, making freshwater and mesosaline lakes most vulnerable to the impacts of climate variability. This is particularly important as global circulation models predict future climate warming to have disproportionate negative impacts on hydrologic conditions in this area.     

Management reference points to account for direct and indirect impacts of fishing on marine mammals

Reference points can help implement an ecosystem approach to fisheries management (EAF), by establishing precautionary removal limits for nontarget species and target species of ecological importance. PBR (Potential Biological Removal), developed under the U.S. Marine Mammal Protection Act (MMPA), is a limit for direct mortality for marine mammals, but it does not account for indirect effects of fishing due to prey depletion. I propose a generalization of PBR (called PBR*) to account for plausible changes in marine mammal carrying capacity (ΔK) from prey biomass decline relative to two example benchmarks: SSB_MSY (maximum sustainable yield biomass for all known prey species) or SSB_K (unfished prey biomass). PBR* can help identify when indirect fishing effects (alone, or combination with direct mortality estimates) may stymie MMPA objectives, and could inform catch limit estimates for target species that are also important as marine mammal prey. As a case study, I applied PBR* estimates to evaluate the possible combined direct + indirect effects of fishing on cetaceans in northeastern U.S. waters. Estimated distributions for ΔK were based on fish stock assessments and meta‐analysis of predator‐prey relationships from the mammalian literature. Based on this analysis, increased risk of marine mammal depletion due to indirect fishing effects was not evident, although this result must be interpreted cautiously given our limited understanding of cetacean diets and marine trophic dynamics. This study is intended to illustrate a possible practical approach for incorporating indirect fisheries impacts on marine mammals into a comprehensive management framework, and it raises several scientific and policy issues that merit further investigation.