P. J. Bacon

Phenological sensitivity to climate across taxa and trophic levels

Differences in phenological responses to climate change among species can desynchronise ecological interactions and thereby threaten ecosystem function. To assess these threats, we must quantify the relative impact of climate change on species at different trophic levels. Here, we apply a Climate Sensitivity Profile approach to 10,003 terrestrial and aquatic phenological data sets, spatially matched to temperature and precipitation data, to quantify variation in climate sensitivity. The direction, magnitude and timing of climate sensitivity varied markedly among organisms within taxonomic and trophic groups. Despite this variability, we detected systematic variation in the direction and magnitude of phenological climate sensitivity. Secondary consumers showed consistently lower climate sensitivity than other groups. We used mid-century climate change projections to estimate that the timing of phenological events could change more for primary consumers than for species in other trophic levels (6.2 versus 2.5–2.9 days earlier on average), with substantial taxonomic variation (1.1–14.8 days earlier on average).

Process-based modelling of decadal trends in growth, survival, and smolting of wild salmon (Salmo salar) parr in a Scottish upland stream

This paper reports a new model of the freshwater stages of an anadromous fish, at the core of which is a stochastic description of the size-at-age dynamics of a growing cohort. Emigration is assumed to require the individual to exceed a threshold size at a critical time of year, thus making the distributions of survival to, and age at, smolting emergent properties of the model. The model is applied to a long-term data set on juvenile Atlantic salmon (Salmo salar) in the Girnock Burn, Scotland, to understand the role played by decadal temperature trends in generating changes in smolt production and age distribution. We conclude that changes in age at smolting are compatible with causation by shifts in the temperature regime. However, the large attenuation between a dramatic fall in spawner numbers and a relatively minor diminution in total smolt production does not result from the physiological effects of temperature but is rather a result of strongly density-dependent mortality between the deposition of o...

An approach to assessing hydrological influences on feeding opportunities of juvenile atlantic salmon (Salmo salar) : a case study of two contrasting years in a small, nursery stream

This case study sought to examine how temporal variability in hydrological and hydraulic conditions might affect the feeding opportunities of juvenile Atlantic salmon in two hydrologically contrasting years of 2002 and 2003, which were characterised by high and low flows respectively. Firstly, measures of hydraulic influence were calculated to define what might be ecologically meaningful disturbance periods during high flows. Secondly, for identifying such periods, the parameter Critical Displacement Velocity (CDV) was derived from a river discharge time series as a first approximation of the amount of time in two hydrologically extreme years when fish foraging strategies for specific age classes of juvenile Atlantic salmon (Salmo salar) might be disrupted by flows. The CDV estimates the threshold velocity above which juvenile salmon are unable to hold station and it is dependent upon fish size and stream temperature. In the wet year 2002, the CDV was exceeded on 18% and 21% of days for 0+ and 1+ fish respectively. In 2003 these respective numbers fell to 6% and 15%. The data suggest that hydrological conditions during certain times of the year have the potential to affect foraging behaviour. This in turn might have implications for recruitment and growth rates for juvenile salmon in upland streams.

Does diurnal temperature variability affect growth in juvenile Atlantic salmon Salmo salar

This study investigated the effects of diurnal temperature variability (>7° C) on the growth of 1+ year Atlantic salmon Salmo salar. Experimental manipulation of water temperature was used to simulate: (1) constant and (2) naturally varying thermal regimes with similar daily mean values. Data from two replicates of four treatments (two thermal and two feeding regimes) were collected over 6 months corresponding to the main spring to summer growth period. Fish growth was assessed at fortnightly intervals. Small but significant differences in mean fork length (L(F) ) and mass were observed between temperature treatments, with smaller, lighter fish under the variable temperature regime. The effects of temperature regime on growth were independent of food ration. At termination of the experiment, the median L(F) and mass of fish exposed to the variable temperature regime were estimated, respectively, to be 2· 6 and 8· 0% less than those under the constant regime. Given the relatively small differences in growth attributable to variable temperature regime in these experiments, it is suggested that mean daily temperatures are adequate to inform juvenile growth models for field-based studies.

Can Conservation Stocking Enhance Juvenile Emigrant Production in Wild Atlantic Salmon

AbstractConservation stocking is frequently used by fishery managers to stabilize or increase production of depleted fish stocks. However, in the case of salmonids the benefits are increasingly questioned and generally poorly quantified. We investigated the effects of ova stocking on freshwater emigrant production in a declining Scottish population of “spring” Atlantic Salmon Salmo salar. The stocking program was designed to minimize risks, maximize expected benefits, and bypass population bottlenecks between the ova and fry stages. Long-term data (33 cohorts over 42 years, including 8 years of stocking) on numbers of ova and juvenile emigrant production were used to investigate whether the ova–emigrant stock–recruitment relationship differed between conditions of natural spawning and stocking. We considered Ricker and Beverton–Holt models that included terms for the effects of stocking, intercohort competition, and changes in trap efficiency on emigrant production. The “best model” was considered to be t...

Modelling length-at-age variability under irreversible growth

In this paper, we describe a discrete-time formalism for describing the dynamics of the size-at-age distribution of a cohort of individuals exhibiting irreversible von Bertalanffy growth in a statistically uniform random environment. This formalism yields a highly efficient numerical implementation, which is particularly suited to automatic optimization. In the special case where mortality is sufficiently size-independent not to vary substantially across the bulk of the size distribution at any given age, we can further increase this efficiency by deriving compact update rules for the mean and coefficient of variation of size-at-age. In this case, we also demonstrate that the depensatory effect of random growth variability and the compensatory effect of deterministic von Bertalanffy growth balance to yield an attracting (initial condition independent) trajectory of mean length and length coefficient of variation against age. We demonstrate the applicability and extensibility of this formalism by two exemplary applications - juvenile salmonids and demersal cod.

Form and uncertainty in stock-recruitment relations : observations and implications for Atlantic salmon (Salmo salar) management

This paper reports an investigation of stock–recruitment relations for Atlantic salmon (Salmo salar). We regard these relations as stochastic functions characterized by an expected stock–recruitment relation and deviations from this expectation driven by observational error and uncharacterised environmental variability. We estimate model parameters by standard Bayesian methods. Analysis of the input–output characteristics of segments of the salmon life cycle in the Girnock Burn (Northeast Scotland) reveals two independent regulatory processes, one between ova and fry and the other between fry and smolts. Comparison of stock–recruitment relations for Atlantic salmon in Scotland, Ireland, and Canada, reinforced by an extended series of simulation studies, shows that even when comparatively long time series of high quality data are available, it is frequently difficult to exclude the possibility of low stock depensation — an effect whose implication of enhanced extinction risk implies that precautionary mana...

Sea-Age Variation in Maiden Atlantic Salmon Spawners: Phenotypic Plasticity or Genetic Polymorphism?

Atlantic salmon exhibit a partially heritable polymorphism in which the morphs are distinguished by the duration and location of the sea-phase of their life-cycle. These morphs co-occur, albeit in characteristically different proportions, in most Scottish rivers and in both the spring and autumn spawner runs; early running fish being generally associated with upland spawning locations while late running fish are associated with lowland spawning. Thus, differences in riverine and marine environment appear to be linked to differences in the relative abundance of the morphs, rather than to the specific morph which is optimally adapted. In this paper, we report a model-based synthetic study aimed at understanding the key dynamic elements which determine the long-term stability of this polymorphism, and thus determine the relative abundance of the various sea-age morphs. Given the recent accumulation of evidence for a genetic basis for the polymorphism, we argue that the key dynamic mechanism which equalises the realized fitness of the sea-age morphs must be one or more morph-specific density dependencies in the riverine phase of the life-history. We explore a number of specific mechanisms, firmly based in known salmon biology, by which such morph-specific density dependence could occur and investigate the robustness of the co-existence which they imply. We conclude that the co-occurrence of multiple sea-age morphs of Atlantic salmon in Scottish rivers is a stable genetic polymorphism, maintained by some combination of physical separation and asymmetric competition between spawners of different morphs or the riverine stages of their offspring or both.

Ova fecundity in Scottish Atlantic salmon Salmo salar: predictions, selective forces and causal mechanisms

Ova fecundities of Scottish Atlantic salmon Salmo salar, predicted from log(10) regression of ova numbers and female fork length (L(F)), differed widely between upland and lowland stocks within the same river, whereas sea-age, river and year factors had insignificant effects on fecundity once L(F) was accounted for. For upland fish, the relationship between log(10)L(F) and log(10) ova mass (M(O)) was stable between two datasets collected 40 years apart. Although upland and lowland females both produced comparable log(10)M(O) (log(10)L(F))(-1), lowland females partitioned this into 45% more, but smaller ova, whereas upland females produced fewer, but larger, eggs. The possible causes and implications of this are discussed for evolutionary perspectives (lifetime production), population structure (local tributary v. large catchments; environmental effects), population dynamics and stability (density-dependent control mechanisms) and fisheries management (stock-recruitment; short and long-term stock sustainability).

Female Length and Mass Quantify the Difference between Atlantic Salmon Ova Fecundity and Ova Deposition

Abstract The population status of salmonids is often assessed as ova deposition relative to “ova conservation densities.” Traditional approaches to assessing ova deposition predict log(ova numbers) from log(female length) for Atlantic salmon Salmo salar based on calibrations for unspawned females. Better ova deposition predictions are obtained by using both female length and female mass, especially when part-spawned females arrive at or leave local study sites, which are crucial to parameterizing such relationships. Improved estimates of ova deposition could help reduce the noise in crucial stock–recruitment relationships and thus improve both mortality estimates at other life stages and management understanding. Received August 23, 2011; accepted January 12, 2012