Lisa A. Kerr

Slow adaptation in the face of rapid warming leads to collapse of the Gulf of Maine cod fishery.

Several studies have documented fish populations changing in response to long-term warming. Over the past decade, sea surface temperatures in the Gulf of Maine increased faster than 99% of the global ocean. The warming, which was related to a northward shift in the Gulf Stream and to changes in the Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation, led to reduced recruitment and increased mortality in the region’s Atlantic cod (Gadus morhua) stock. Failure to recognize the impact of warming on cod contributed to overfishing. Recovery of this fishery depends on sound management, but the size of the stock depends on future temperature conditions. The experience in the Gulf of Maine highlights the need to incorporate environmental factors into resource management. Warming waters prevented cod recovery in the North Atlantic. Double jeopardy In the best of worlds, exploited fish stocks are monitored so that harvest quotas protect the reproductive ability of the population. Climate change is likely to complicate this process substantially. Pershing et al. found that cod stocks declined continuously during intense warming in the North Atlantic. Fisheries quotas, even though they were responsibly set and followed by fishers, decreased the reproductive rate. Thus, managing fisheries in a warming world is going to be increasingly problematic. Science, this issue p. 809

The role of spatial dynamics in the stability, resilience, and productivity of an estuarine fish population

Understanding mechanisms that support long-term persistence of populations and sustainability of productive fisheries is a priority in fisheries management. Complex spatial structure within populations is increasingly viewed as a result of a plastic behavioral response that can have consequences for the dynamics of a population. We incorporated spatial structure and environmental forcing into a population model to examine the consequences for population stability (coefficient of variation of spawning-stock biomass), resilience (time to recover from disturbance), and productivity (spawning-stock biomass). White perch (Morone americana) served as a model species that exhibits simultaneous occurrence of migratory and resident groups within a population. We evaluated the role that contingents (behavioral groups within populations that exhibit divergent life histories) play in mitigating population responses to unfavorable environmental conditions. We used age-structured models that incorporated contingent-specific vital rates to simulate population dynamics of white perch in a sub-estuary of Chesapeake Bay, USA. The dynamics of the population were most sensitive to the proportion of individuals within each contingent and to a lesser degree to the level of correlation in recruitment between contingents in their responses to the environment. Increased representation of the dispersive contingent within populations resulted in increased productivity and resilience, but decreased stability. Empirical evidence from the Patuxent River white perch population was consistent with these findings. A high negative correlation in resident and dispersive contingent recruitment dynamics resulted in increased productivity and stability, with little effect on resilience. With high positive correlation between contingent recruitments, the model showed similar responses in population productivity and resilience, but decreased stability. Because contingent structure involves differing patterns of nursery habitat use, spatial management that conserves sets of habitats rather than the single most productive nursery habitat would be expected to contribute to long-term population stability.

Partial Migration of Fishes as Exemplified by the Estuarine-Dependent White Perch

Abstract Partial migration defines the phenomenon of coexisting groups exhibiting migratory and resident behavior within the same population. In avian ecology, partial migration is a fundamental behavior that underlies the evolution of migration in general. Among fish taxa, the terminology and theory associated with partial migration has predominantly been used to describe salmon ecophenotypes, but the behavior is more widespread. Here, we ecophenotypes, but the behavior is more widespread. Here, we document partial migration for the estuarine-dependent white perch (Morone americana), in the Patuxent River estuary (Chesapeake Bay, Maryland), wherein a portion of the population resides in freshwater natal habitats and another portion migrates down-estuary into brackish water (salinities>3) habitats. Life-time migration histories were examined using otolith strontium:calcium profile analysis. Alternative life history tactics, initiated during the juvenile period in response to individual physiological condi...

Investigations of Δ14C, δ13C, and δ15N in vertebrae of white shark (Carcharodon carcharias) from the eastern North Pacific Ocean

The white shark, Carcharodon carcharias, has a complex life history that is characterized by large scale movements and a highly variable diet. Estimates of age and growth for the white shark from the eastern North Pacific Ocean indicate they have a slow growth rate and a relatively high longevity. Age, growth, and longevity estimates useful for stock assessment and fishery models, however, require some form of validation. By counting vertebral growth band pairs, ages can be estimated, but because not all sharks deposit annual growth bands and many are not easily discernable, it is necessary to validate growth band periodicity with an independent method. Radiocarbon (14C) age validation uses the discrete 14C signal produced from thermonuclear testing in the 1950s and 1960s that is retained in skeletal structures as a time-specific marker. Growth band pairs in vertebrae, estimated as annual and spanning the 1930s to 1990s, were analyzed for Δ14C and stable carbon and nitrogen isotopes (δ13C and δ15N). The aim of this study was to evaluate the utility of 14C age validation for a wide-ranging species with a complex life history and to use stable isotope measurements in vertebrae as a means of resolving complexity introduced into the 14C chronology by ontogenetic shifts in diet and habitat. Stable isotopes provided useful trophic position information; however, validation of age estimates was confounded by what may have been some combination of the dietary source of carbon to the vertebrae, large-scale movement patterns, and steep 14C gradients with depth in the eastern North Pacific Ocean.

Chemical Composition of Fish Hard Parts as a Natural Marker of Fish Stocks

The chemical composition of calcified structures in fish (otoliths, spines, rays, scales, and vertebrae) has been identified as a useful natural marker of stock structure. This application depends on geographic variation in water chemistry or other factors (e.g., temperature) that influences the chemistry of the calcified structure such that fish that inhabit different environments exhibit differences in their chemical signature. The use of this technique in stock discrimination requires that a number of assumptions must be met, including that groups of interest possess characteristic and reproducible markers and all possible groups contributing to the group mixture have been characterized. In some instances, structural chemistry can permit observation of subtle population structure that may not be detected by genetic applications alone. Chemical analysis of calcified structures is most effective when used as part of a holistic approach to stock identification.

Bomb radiocarbon and lead-radium disequilibria in otoliths of bocaccio rockfish (Sebastes paucispinis): a determination of age and longevity for a difficult-to-age fish

Longevity estimates for the bocaccio rockfish (Sebastes paucispinis) using traditional techniques range from less than 20 years to approximately 50 years. Otoliths of bocaccio are difficult to age, and previous attempts to validate ages have been unsuccessful. Because otolith age suggests the bocaccio are reasonably long-lived, lead-radium dating was used in an attempt to independently age bocaccio otoliths. The measured 210 Pb and 226 Ra activities were among the lowest reported and resulted in poor radiometric age resolution; however, the break- and-burn technique clearly underestimated age in some cases with the longevity of the bocaccio being at least 31 years. To provide better age resolution, the bomb radiocarbon approach was applied to individual otoliths. Based on measured radiocarbon levels relative to a reference time-series, several specimens were aged at approximately 30-40 years. To evaluate these determinations, the remaining otolith of the pair was sectioned and aged blind. The result was an excellent fit to the reference time-series and a validation of the age estimates. The maximum age from growth zone counts was 37 ± 2 years, which is consistent with a reported maximum age of approximately 50 years.

Investigations of age and growth for three deep-sea corals from the Davidson Seamount off central California

A recent biological characterization of the Davidson Seamount off central California using a remotely operated vehicle revealed communities rich with deep-sea corals. During this characterization several corals were collected and three colonies were made available for an age and growth study. The colonies examined were identified as bubblegum coral (Paragorgia sp.), bamboo coral (Keratoisis sp.), and precious coral (Corallium sp.). Age was estimated from growth zone counts made in skeletal cross sections. These age estimates were used to estimate growth rates and colony age. Estimated growth rates determined for each species were quite different. The bubblegum coral had a relatively high estimated growth rate, with the precious and bamboo coral estimated as slow growing. These age and growth observations were evaluated relative to published studies on related species and an attempt was made to validate the age and growth estimates with an independent radiometric ageing technique (i.e., lead-210 dating). This approach was not successful for the bubblegum coral, and was successful for the bamboo and precious corals to differing degrees. For the bamboo coral, a minimum colony age of over 200 years was determined. For the precious coral, a linear growth rate of approximately 0.25 cm/yr led to a colony age of about 115 years; however, based on the radial growth rate, an age of up to 200 year is possible.

Bioenergetic trajectories underlying partial migration in Patuxent River (Chesapeake Bay) white perch (Morone americana)

Partial migration, the coexistence of resident and migratory individuals within the same population, may be common in fish populations. A proposed mechanism underlying partial migration is differing dispersive responses to early growth conditions, but few studies have explicitly tested this. During their first year of life, white perch (Morone americana) in the Patuxent River (Maryland, USA) exhibit either residency in freshwater natal habitats (resident contingent) or disperse down-estuary into brackish habitats (dispersive contingent). We tested whether white perch juveniles exhibited differing growth and metabolic trajectories based on contingent membership or in response to salinity. A randomized factorial experiment with two contingent types and two salinity treatments (1 and 8) was conducted over a 30-day period. The experiments supported a contingent effect, with the dispersive contingent exhibiting higher consumption rates and a higher scope for growth. In addition, we identified a weak salinity e...

Age validation of canary rockfish (Sebastes pinniger) using two independent otolith techniques: lead-radium and bomb radiocarbon dating

Canary rockfish (Sebastes pinniger) have long been an important part of recreational and commercial rockfish fishing from south-east Alaska to southern California, but localised stock abundances have declined considerably. Based on age estimates from otoliths and other structures, lifespan estimates vary from ~20 years to over 80 years. For the purpose of monitoring stocks, age composition is routinely estimated by counting growth zones in otoliths; however, age estimation procedures and lifespan estimates remain largely unvalidated. Typical age validation techniques have limited application for canary rockfish because they are deep-dwelling and may be long-lived. In this study, the unaged otolith of the pair from fish aged at the Department of Fisheries and Oceans Canada was used in one of two age validation techniques: (1) lead-radium dating and (2) bomb radiocarbon (14C) dating. Age estimate accuracy and the validity of age estimation procedures were validated based on the results from each technique. Lead-radium dating proved successful in determining that a minimum estimate of lifespan was 53 years and provided support for age estimation procedures up to ~50–60 years. These findings were further supported by Δ14C data, which indicated that a minimum estimate of lifespan was 44 ± 3 years. Both techniques validate, to differing degrees, age estimation procedures and provide support for inferring that canary rockfish can live more than 80 years.

Partial Migration Across Populations of White Perch (Morone americana): A Flexible Life History Strategy in a Variable Estuarine Environment

We evaluated the prevalence of partial migration, coexisting resident and migratory life history types, within six white perch (Morone americana) populations in sub-estuaries (Upper Bay, and Potomac, Choptank, Nanticoke, James, and York Rivers) of the Chesapeake Bay. Otolith stable isotope (δ18O) values were used to resolve fish habitat use along an estuarine salinity gradient and define resident or migratory behavior. The majority of adults within Upper Bay and Potomac River populations were resident, whereas individuals from the Choptank, Nanticoke, James, and York Rivers were predominantly migratory. Beyond population differences, large interannual variability in life history types was observed, likely due to differences in estuarine conditions that influence growth rate of individuals (e.g., temperature, zooplankton density). Because we observed partial migration in all study populations, we suggest that this trait is characteristic of this species, permitting plastic responses to variation in the estuarine environment.