Life in the Slow Lane: Ecology and Conservation of Long-Lived Marine Animals

<em>Abstract.</em> —The sandbar shark <em>Carcharhinus plumbeus </em> is the most important species caught in the commercial shark fishery operating off the U.S. Atlantic and Gulf of Mexico coasts. Previous demographic studies of this and other species of sharks have utilized age-structured, deterministic life tables that provided point estimates of maximum rates of increase. To reduce some of the uncertainty in estimates of age at maturity and longevity—especially acute in the case of the sandbar shark—I constructed a stage-based model based on an Usher matrix that utilizes the more reliable estimates of size at maturity and maximum size for this species in the northwest Atlantic. Because demographic variability also can affect estimated rates of increase, I introduced stochasticity into the model by randomly selecting fecundity rates from an empirically determined distribution, and natural mortality rates from estimates obtained through four life history methods. The simulation model was applied to females only. Population projections 20 years forward in time without exploitation predicted slowly growing populations at approximately 1.3%/year. Application of a constant instantaneous mortality rate (<EM>F</EM> ) of 0.1 to each stage-class separately indicated that removal of large juveniles would produce the greatest population declines, whereas removal of age-0 individuals would be sustainable. The simulation model was then used to predict potential outcomes under three hypothetical harvesting scenarios using the current U.S. commercial quota indicating that all strategies produced pronounced population declines.

Life in the Slow Lane: Ecology and Conservation of Long-Lived Marine Animals

<em>Abstract.</em> —Radiometric aging of fishes is a recently developed technique using the disequilibria of ²¹⁰ Pb:²²⁶ Ra in calcified structures to determine age. It has been applied successfully to several fish species, but certain limitations have made improvements desirable. Because ²²⁶ Ra can be measured directly by counting atoms using isotope-dilution thermal ionization mass spectrometry (TIMS), a new ion-exchange separation technique was developed to isolate small quantities of radium from calcified structures for TIMS determination. The advantages of this new technique are reduced sample mass and processing time, and greater accuracy and precision of radium quantification. We applied this technique to calcified structures from three fish species: otolith cores of Pacific grenadier <em>Coryphaenoides acrolepis </em> and tarpon <em>Megalops atlanticus, </em> and pectoral fin ray cores of Atlantic sturgeon <em>Acipenser oxyrinchus. </em> Annulus-derived age estimates for <em>C. acrolepis </em> were accurate with a confirmed longevity of at least 48 years. Although annulus-derived ages for <em>M. atlanticus </em> were inconsistent with radiometric ages, radiometric aging confirmed that tarpon are longlived; females may exceed 82 years. Radiometric age could not be determined for <em>A. oxyrinchus </em> because ²¹⁰ Pb activities were greater than could be supported by ingrowth from ²²⁶ Ra decay. In this paper we discuss the application and limitations of this technique and its relevance to fisheries management.

Life in the Slow Lane: Ecology and Conservation of Long-Lived Marine Animals

<em> Abstract.</em>—Sharks have the reputation of being particularly vulnerable to fishing pressure, a fact attributed to their generally ‘<EM> K</EM>-selected’ life history strategies. The history of shark fisheries is not encouraging, and their poor record of sustainability is compounded by the fact that few countries have any form of management for these resources. The Australian Southern Shark Fishery provides an example of a well-studied shark fishery with a long history of exploitation that has been under a management plan for some ten years. This fishery is unique in that it exploits, under a similar fishing regime, two similar target shark species that show very different responses to fishing pressure. Stock assessments suggest that under current fishing effort the catch of one species is sustainable while the other species is overexploited. The vulnerability of the two species to the fishing gear is similar but their biological productivity is very different. The selective pressures that may have given rise to these different life history strategies are discussed.

Life in the Slow Lane: Ecology and Conservation of Long-Lived Marine Animals

<em>Abstract.</em> —The spatial and seasonal distribution of humpback whales in the Great Barrier Reef Marine Park (GBRMP) was defined using data from a systematic aerial surveillance program. The data comprised 414 pod sightings (812 individuals) recorded from July 1982 to March 1996. These sightings were supposedly of humpbacks from the east Australian Group V substock that migrates during the austral autumn from Antarctic feeding grounds to winter breeding grounds in GBR waters. Humpbacks were sighted in all months and throughout the GBRMP. However, most pods (75%) were sighted in southern GBR waters (below 19°S) and mainly during winter and spring ( July to September). Occasional sightings of humpbacks in northern GBR waters (above 16°S) in summer supports previous claims of a substock resident year-round in northern Australian tropical waters. Mother–calf sightings were rare with most recorded below 21°S and mainly in August and September. These limited sightings suggest that the main calving grounds for the east Australian Group V substock occur in the extensive southern GBR lagoonal waters defined northward by the Whitsunday Group of islands and reefs and eastward by the Pompey/Swains reef complex. An estimate of the crude birth rate was 0.072 (95% confidence interval [CI]: 0.06–0.11) with Monte Carlo estimates of the median calving rate at 0.3 calves per mature female per year (95% CI: 0.22–0.43) and the median interbirth interval at 3.4 years (95% CI: 2.3–4.5) indicating low and variable juvenile recruitment. Nonparametric time series analysis (seasonal and trend decomposition using loess, STL) of monthly humpback sightings showed that the long-term trend in sightings was increasing but that there was significant inter-annual variability in the seasonal abundance of humpbacks in the GBRMP. The STL analysis also suggested that the frequency of sightings increased earlier in winter (June) and later in the season during spring/summer (October to December). Time series regression analysis of the STL-derived trend in sightings suggested that the east Australian Group V substock increased slowly in abundance over the 14 years from 1982 to 1996 at about 3.9% per year (95% CI: 1.9% to 5.7%)—a finding consistent with an estimate of low and variable juvenile recruitment.

Life in the Slow Lane: Ecology and Conservation of Long-Lived Marine Animals

<em>Abstract.</em> —Long-lived marine species are particularly vulnerable to human perturbations for reasons related both to the species’ demographics and human perception. Marine turtles provide an illustrative example: species delay maturity from 10 to as late as 30–60 years, and human monitoring of turtle populations focuses primarily on nesting adult females and nest production. Apparently marine turtles are trading early reproduction for later reproduction at a larger size, facilitating higher fecundity to overcome variable, often very high, natural mortality in eggs and early life stages. Human perturbations increase mortality of marine turtles at all life stages. However, population modeling has shown that annual survival of some stages, particularly large juveniles and adults, may be particularly critical to population maintenance and recovery. At the same time, monitoring focused on the larger, usually older, life stages of long-lived marine species may be a relatively poor indicator of actual population health and trends. High bycatch of long-lived marine species in long-line fisheries is outlined and concerns raised. Precautions, based on long-lived species’ vulnerability and the limitations of our ability to adequately monitor population status, should be built into management regimes.

Life in the Slow Lane: Ecology and Conservation of Long-Lived Marine Animals

<em>Abstract.</em> —This manuscript summarizes the results of seven studies of population genetics in sharks and several other studies that estimated levels of genetic variation in sharks. Investigations of population genetics in sharks are rare relative to studies in other fishes, and no analyses have been performed on any member of most orders of chondrichthyans. Allozymes and mitochondrial DNA typically reveal low levels of genetic variation within sharks, perhaps due in part to the demographic characteristics of sharks. Levels of genetic variation are significantly lower than those in marine teleosts. Sharks exhibit little genetic heterogeneity across wide geographic ranges; however, some stock structure has been detected both within and between oceans. Molecular characters have been used to confirm the presence of cryptic species and to support the synonymization of allopatric populations under a single species name.

Life in the Slow Lane: Ecology and Conservation of Long-Lived Marine Animals

<em>Abstract.</em> —The history of the Hawaiian monk seal is one of remarkable evolutionary persistence but recent and severe decline. The species may have persisted in the Hawaiian Archipelago for the past 15 million years. Its initial contact with humans probably occurred when Polynesians first settled the Archipelago, perhaps 2,000 years ago. Shortly thereafter, the monk seal was likely extirpated in the main Hawaiian Islands and reduced to its current configuration of six main reproductive colonies. The first written record of the species is from 1805, when it was discovered on Lisianski Island. Through the remainder of the 1800s, it was exploited for skins and oil and for food by shipwrecked sailors. By the turn of the century, at least two of the six reproductive colonies had been extirpated and the species was severely depleted. In this century, the Hawaiian monk seal has suffered more from habitat competition and loss than from direct exploitation. Habitat loss has occurred in the form of permanent human settlement with associated disturbance, deposition of marine debris, physical and biological alteration of its terrestrial environment, and biological alteration of its marine environment. Thus, human activities have played a major role in determining the status and trends of the species over the past two centuries. The historical record also suggests that human access to the Northwestern Hawaiian Islands is increasing, and such activities may become a greater impediment to recovery if they are not limited to those compatible with wildlife conservation. Unfortunately, we are often unable to predict the compatibility of proposed activities. In the face of such uncertainty, is further development of the Northwestern Hawaiian Islands worth the risk of extinction of the Hawaiian monk seal?

Life in the Slow Lane: Ecology and Conservation of Long-Lived Marine Animals

<em>Abstract.</em> —A suite of stage- and age-classified models are constructed to determine whether an apparently unusual event is the result of external, environmental causes in a small population. These models are used as a baseline, or null hypothesis, that such an event may result from population structure and demographic stochasticity. An observed multiyear reproductive delay in a pod of killer whales <em>Orcinus orca </em> is used as an example of this process. All models, regardless of their complexity, give the same qualitative result: the observed reproductive delay could not be explained by pod composition and demographic stochasticity; external causes have to be sought to explain this phenomenon.

Life in the Slow Lane: Ecology and Conservation of Long-Lived Marine Animals

<em>Abstract.</em> —Seabirds become mature at a late age, experience low annual fecundity, often refrain from breeding, and enjoy annual adult survival rates as high as 98%. This suite of life history characteristics limits the capacity for seabird populations to recover quickly from major perturbations, and presents important conservation challenges. Concern over anthropogenic impacts on seabird populations has led to the initiation of long-term field programs to monitor seabird reproductive performance and population dynamics. In addition, seabirds have been recognized as potentially useful and economical indicators of the state of the marine environment and, in particular, the status of commercially important prey stocks. This paper reviews demographic and life history attributes of seabird populations and uses this information to explore the consequences of longevity from the respective standpoints of conservation and monitoring goals. Analysis of a simplified life cycle model reveals that maximum potential population growth rates (λ) under ideal circumstances fall within the range of 1.03–1.12 for most species, though growth rates realized in nature will always be lower. Elasticity analysis confirms that seabird population growth rates are extremely sensitive to small variations in adult survival rates, and dictates that survival monitoring should be considered an essential component of conservation strategies. As in other organisms with long life spans, ecological and physiological costs of reproduction are expected to figure prominently in seabird reproductive decisions. Consequently, understanding how seabirds allocate reproductive effort in response to varying environmental conditions is an important prerequisite for correctly interpreting field data from monitoring studies.