Natural Age Structure Research Articles

Optimal harvest regulations under conflicting tradeoffs between conservation and recreational fishery objectives

Length-based harvest regulations alter the fishing-induced demographic and evolutionary trajectories of exploited stocks and thus shape the existing tradeoffs among fishery and conservation objectives. We used a structurally realistic eco-genetic individual-based harvest model that implements dynamic angling mortality and cryptic mortality sources (illegal harvest and hooking mortality). We (1) analyzed the effects of alternative length-based harvest regulations under scenarios involving different combinations of exploitation intensity and hooking mortality on a suite of indicators of fishery performance and conservation status of a freshwater fish stock, and (2) determined the regulations that optimize the tradeoff among selected indicators under different management strategies, and fishery and conservation objectives. Fishing scenarios under a maximum-length limit regulation maximized harvest yield but led always to recruitment overfishing, irrespective of the exploitation and hooking mortality rates simulated. Fishing scenarios under a harvest slot limits regulation (HS) were best at maintaining a high status of old, large, fecund fish and a more natural age-structure with higher biomass and reproductive potential, performing increasingly better than minimum-length limit (MLL) regulations with decreasing hooking mortality. Both regulation types were effective at preventing overexploitation and only under scenarios with low restrictiveness and high exploitation intensity and hooking mortality was the stock at risk of recruitment overfishing. MLLs outperformed HS regulations in terms of fishery performance, consistently presenting greater harvest yield and efficiency, and size of harvested fish. High rates of hooking mortality rendered HS regulations less effective than assumed, so they were always outperformed by MLLs irrespective of the management strategy and objectives. When hooking mortality was low, HSs constituted the optimal regulation type in most cases except when high fishery performance was favoured over conservation objectives or harvest of large fish was regarded as critically important.

Stand-replacing wildfires?: The incidence of multi-cohort and single-cohort Eucalyptus regnans and E. obliqua forests in southern Tasmania

The natural age structure of wet eucalypt forest has important implications for biodiversity conservation and the mode of wood production. Southern Tasmanian wet eucalypt forests were sampled to describe age class variation and to test the following hypotheses that relate to it: (1) Eucalyptus regnans stands are more likely to be single-cohort than Eucalyptus obliqua and mixed stands; (2) old-growth trees are associated with multi-cohort wet eucalypt forests; (3) the E. regnans stands are more multi-cohort than Victorian E. regnans stands. Data from 762 stands, all with either E. obliqua or E. regnans were analysed to determine how stand characteristics related to forest type and to the presence of old-growth trees. Over half the stands studied were multi-cohort. Stands with E. regnans had a lesser tendency towards multi-cohortness than stands lacking this species, although most old-growth stands, including those dominated by E. regnans, were multi-cohort. In contrast, most regrowth stands of all species combinations were single-cohort. The proportions of E. regnans stands that were multi-cohort were similar to some estimates from the same type of forest in Victoria. Modifications of forestry regimes in wet eucalypt forests could help to maintain the existence of these biodiverse multi-cohort forests in the landscape.

Aging and demographic plasticity in response to experimental age structures in honeybees (Apis mellifera L)

Honeybee colonies are highly integrated functional units characterized by a pronounced division of labor. Division of labor among workers is mainly age-based, with younger individuals focusing on in-hive tasks and older workers performing the more hazardous foraging activities. Thus, experimental disruption of the age composition of the worker hive population is expected to have profound consequences for colony function. Adaptive demography theory predicts that the natural hive age composition represents a colony-level adaptation and thus results in optimal hive performance. Alternatively, the hive age composition may be an epiphenomenon, resulting from individual life history optimization. We addressed these predictions by comparing individual worker longevity and brood production in hives that were composed of a single age cohort, two distinct age cohorts, and hives that had a continuous, natural age distribution. Four experimental replicates showed that colonies with a natural age composition did not consistently have a higher life expectancy and/or brood production than the single cohort or double cohort hives. Instead, a complex interplay of age structure, environmental conditions, colony size, brood production, and individual mortality emerged. A general trade-off between worker life expectancy and colony productivity was apparent, and the transition from in-hive tasks to foraging was the most significant predictor of worker lifespan irrespective of the colony age structure. We conclude that the natural age structure of honeybee hives is not a colony-level adaptation. Furthermore, our results show that honeybees exhibit pronounced demographic plasticity in addition to behavioral plasticity to react to demographic disturbances of their societies.

Multiple paternity is prevalent in Pacific ocean perch (Sebastes alutus) off the Oregon coast, and is correlated with female size and age

The need to rebuild Pacific ocean perch, Sebastes alutus, populations on the west coast of the United States has precipitated a need to better understand the life history characteristics of this rockfish species. One such characteristic is mating behavior, which has the potential to influence the amount of genetic diversity in a population. We documented and examined the frequency of multiple mating in Pacific ocean perch collected off the Oregon coast using five microsatellite loci. We found that 47 of 66 (71.2%) females examined had broods sired by multiple males. The mean number of sires per brood was 1.92 (SD = 0.76) and ranged from 1-4. Polyandrous females were significantly larger and had an older average age than monogamous females. Our results suggest that polyandrous behavior among female Pacific ocean perch off the coast of Oregon is prevalent, is related to female size and age, and should be preserved by maintaining a natural age structure in this population.

Survival of Rhyzopertha dominica (Coleoptera: Bostrichidae) in Stored Wheat Under Fall and Winter Temperature Conditions

Cold temperature survivorship of Rhyzopertha dominica (F.) populations with a natural age structure in stored wheat was estimated for a natural fall cooling rate of 2 degrees C per week and winter temperatures of 17, 13, 9, and 5 degrees C. Logistic equations were fitted to the data to predict the survivorships of the adults outside kernels and immatures and preemergent adults inside kernels as a function of exposure time over the 5-17 degrees C temperature range. During the acclimation period, many insects died, and roughly 80, 55, and 25% of the insects were alive after temperatures had decreased at a rate of 2 degrees C per week from 17 degrees C to 13, 9, and 5 degrees C, respectively. At the end of cold temperature exposure period, many R. dominica survived at 17 degrees C, a few at 13 degrees C, but none at 5 or 9 degrees C. The colder the temperature the more rapidly survivorship decreased. The mostly immature insect population inside kernels was killed less rapidly than the external adult population at 5 and 9 degrees C and more rapidly at 13 and 17 degrees C. Daily survival rates decreased and then increased again indicating that those insects surviving short exposures to cold temperatures had a greater chance of surviving long exposures to cold temperatures. These equations will help extend our population model to predict the survival of R. dominica populations in stored wheat through the winter.