Unobserved Mortality Research Articles

Unobserved mortality occurs early in larval walleye (Sander vitreus) aquaculture

One of the most persistent challenges in walleye larviculture is the high rate of unobserved mortality occurring within the first 30 days post-hatch. Walleye larviculture is characterized by high mortality rates, but oftentimes, only a fraction of the total mortality in walleye larviculture is observed. Unobserved mortality is frequently attributed to a high rate of cannibalism, but decomposition of larvae and other factors may also be important contributors. Unobserved mortality can make walleye larviculture especially challenging as accurately estimating daily feed rations and forecasting larval production rely on accurate counts of surviving fish. Despite the importance of unobserved mortality, there is little empirical evidence available that assesses the temporal occurrence and causes of unobserved mortality in walleye larviculture. This study assessed when unobserved mortality occurs during walleye larviculture by examining survival at three timepoints (8, 15, and 30 days post-stocking). Tanks were cleaned daily and dead fry were enumerated to account for observed mortality. At the end of each treatment period, the tanks were drained, all remaining fry were enumerated, and the unobserved mortality rate was calculated. While observed mortality trended higher as trial duration increased, unobserved mortality did not increase significantly from the 8-day treatment suggesting that the bulk of unobserved mortality occurs during the first 8 days of walleye larviculture.

BIAS-CORRECTED INFERENCE FOR A MODIFIED LEE–CARTER MORTALITY MODEL

AbstractAs a benchmark mortality model in forecasting future mortality rates and hedging longevity risk, the widely employed Lee–Carter model (Lee, R.D. and Carter, L.R. (1992) Modeling and forecasting U.S. mortality. Journal of the American Statistical Association, 87, 659–671.) suffers from a restrictive constraint on the unobserved mortality index for ensuring model’s identification and a possible inconsistent inference. Recently, a modified Lee–Carter model (Liu, Q., Ling, C. and Peng, L. (2018) Statistical inference for Lee–Carter mortality model and corresponding forecasts. North American Actuarial Journal, to appear.) removes this constraint and a simple least squares estimation is consistent with a normal limit when the mortality index follows from a unit root or near unit root AR(1) model with a nonzero intercept. This paper proposes a bias-corrected estimator for this modified Lee–Carter model, which is consistent and has a normal limit regardless of the mortality index being a stationary or near unit root or unit root AR(1) process with a nonzero intercept. Applications to the US mortality rates and a simulation study are provided as well.

Evaluating off-bottom sweeps of a U.S. West Coast groundfish bottom trawl: Effects on catch efficiency and seafloor interactions

In the U.S. West Coast groundfish bottom trawl fishery, lengthy sweeps (>85 m) that maintain seafloor contact are traditionally used. While these sweeps are effective at herding groundfishes, their bottom tending characteristics increase the potential to cause seafloor disturbances, and injury and unobserved mortality to benthic organisms. In this study, we examined if changing from conventional to modified sweeps (with sections elevated 6.5 cm off bottom) would affect catch efficiency of target groundfishes and seafloor interactions. We used a DIDSON imaging sonar to observe how each sweep configuration interacted with the seafloor. An altimeter was periodically placed on the modified sweep to measure height off bottom. Results detected no significant catch efficiency effect of changing from conventional to modified sweeps. The DIDSON and altimeter data showed the modified sweeps exhibit elevated sections where infaunal and lower-profile epifaunal organisms can pass under without disturbance. Results demonstrate that seafloor interactions can be substantially reduced using elevated sweeps in this fishery without impacting catch efficiency. Further, findings from this research could be potentially applicable to other fisheries nationally and internationally.

Causes of mortality in depleted populations of Atlantic cod estimated from multi-event modelling of mark–recapture and recovery data

Knowledge on mortality causes is key for an effective management of animal populations and can help to restore depleted fish stocks. Here we investigated the mortality dynamics of coastal Atlantic cod (Gadus morhua) in Skagerrak, southern Norway, by analyzing local mark–recapture and recovery data collected from 2005 to 2013 (N = 9360 fish, mean length = 41 cm, range = 16–93 cm). By applying multi-event models to the data, we could link field observations to multiple “dead states” and estimate the proportion of deaths associated with different fishing gears while controlling for unobserved mortality and detection errors. Deaths due to hand lines and fixed gear types were dominant compared with other causes, especially in legal-sized cod (≥40 cm). Gear-specific mortality changed over time and between size classes, but annual survival remained low and stable (∼0.3). Assuming fully additive mortality, we predicted annual survival of cod to be above 0.5 if only one or both of the dominant gear types were removed, providing insights on the relative impact of diverse harvesting practices on local population dynamics.

Quantification and reduction of unobserved mortality rates for snow, southern Tanner, and red king crabs (Chionoecetes opilio, C. bairdi, and Paralithodes camtschaticus) after encounters with trawls on the seafloor

Quantification and reduction of unobserved mortality rates for snow, southern Tanner, and red king crabs (Chionoecetes opilio, C. bairdi, and Paralithodes camtschaticus) after encounters with trawls on the seafloor

Behavioural responses of sardines Sardina pilchardus to simulated purse‐seine capture and slipping

The behavioural effects of confinement of sardine Sardina pilchardus in a purse seine were evaluated through three laboratory experiments simulating the final stages of purse seining; the process of slipping (deliberately allowing fishes to escape) and subsequent exposure to potential predators. Effects of holding time (the time S. pilchardus were held or entangled in the simulation apparatus) and S. pilchardus density were investigated. Experiment 1 compared the effect of a mild fishing stressor (20 min in the net and low S. pilchardus density) with a control (fishing not simulated) while the second and third experiments compared the mild stressor with a severe stressor (40 min in the net and high S. pilchardus density). In all cases, sea bass Dicentrarchus labrax were used as potential predators. Results indicated a significant effect of crowding time and density on the survival and behaviour of slipped S. pilchardus. After simulated fishing, S. pilchardus showed significant behavioural changes including lower swimming speed, closer approaches to predators and higher nearest-neighbour distances (wider school area) than controls, regardless of stressor severity. These results suggest that, in addition to the delayed and unobserved mortality caused by factors related to fishing operations, slipped pelagic fishes can suffer behavioural impairments that may increase vulnerability to predation. Possible sub-lethal effects of behavioural impairment on fitness are discussed, with suggestions on how stock assessment might be modified to account for both unobserved mortality and sub-lethal effects, and possible approaches to provide better estimates of unobserved mortality in the field are provided.

Using reflex action mortality predictors (RAMP) to evaluate if trawl gear modifications reduce the unobserved mortality of Tanner crab (Chionoecetes bairdi) and snow crab (C. opilio)

Abstract Hammond, C. F., Conquest, L. L., and Rose, C. S. 2013. Using reflex action mortality predictors (RAMP) to evaluate if trawl gear modifications reduce the unobserved mortality of Tanner crab (Chionoecetes bairdi) and snow crab (C. opilio). – ICES Journal of Marine Science, 70: . Management of Bering Sea crab and groundfish fisheries must account for the delayed (and hence unobserved) mortality of crab that encounter bottom trawls, but are not captured. A new approach to predicting the delayed mortality of crab uses a reflex action mortality predictor (RAMP) model to establish a relationship between mortality and reflex impairments. A 2007 pilot study of 649 crab established RAMP curves for both Tanner crab (Chionoecetes bairdi) and snow crab (C. opilio). Additional data (1775 crab) collected in 2008 allowed us to update the existing RAMP curves to more fully examine the effect of injury scores on the RAMP models and to determine the best method for estimating overall mortality. Results confirmed that the additional measurements did not significantly alter the original relationship between mortality and reflex impairment score. Additionally, the RAMP curves were used to predict unobserved mortality from observed reflex impairment scores of Chionoecetes spp. captured after encounters with different parts of bottom trawl gear (footrope and sweep) and alternative types of footrope and sweep. In addition to estimating mortality rates caused by each gear part, we tested whether the alternative footrope and sweep designs reduced the unobserved mortality rates of crab. Results showed that the alternative footrope (58 cm disk footrope) reduced mortality from 11.4 to 7.2% for Tanner crab and from 9.7 to 5.0% for snow crab. The alternative (off-bottom) sweep reduced mortality from 4.1 to 1.0% for Tanner crab and from 4.9 to 0.0% for snow crab. Thus, the use of the reflex impairment score through the RAMP model is a cost effective way to estimate delayed mortality and to assess the effect of gear types on delayed mortality for management purposes.

Life or death in the fast lane: dolphin mother‐calf separation during tuna purse‐seine sets

Dolphin stocks in the eastern tropical Pacific Ocean, reduced 70–80% due to fishery‐related mortality 1960–1990, are not recovering despite near‐zero fishery mortality since 1991. Because mortality records show that 75–95% of killed lactating females are not killed with a nursing calf, the fishing process appears to separate mothers and calves, so that unobserved mortality of calves may be important to the lack of stock recovery. A suite of studies investigated this question from a number of perspectives, including review of physiological and behavioral development in dolphin calves, mathematical analysis of dolphin drafting hydrodynamics, and empirical studies of swim development in dolphin calves from birth through two years. In toto, these studies show that the sustained high‐speed swimming and accompanying aerial respirations characteristic of dolphins evading purse‐seine sets creates conditions under which dolphin calves will have great difficulty sustaining a drafting relationship with the mother, and that developmental limits are likely to prevent calves from maintaining an association with the mother while swimming independently. Effects are strongly age‐related, with youngest calves at greatest risk for separation.

Indirect effects of recreational fishing on behavior of the spiny lobster Panulirus argus

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 303:235-244 (2005) - doi:10.3354/meps303235 Indirect effects of recreational fishing on behavior of the spiny lobster Panulirus argus Darren M. Parsons*, David B. Eggleston North Carolina State University, Department of Marine, Earth and Atmospheric Sciences, Raleigh, North Carolina 27695-8208, USA *Email: darren.parsons@clear.net.nz ABSTRACT: Sublethal disturbance may lead to behavioral modifications that have detrimental consequences for the individual. Sublethal disturbance is an indirect effect of fishing that has seldom been examined. In summer 2003, we conducted surveys on the Caribbean spiny lobster Panulirus argus in the Florida Keys, USA, before and after a 2 d mini-season exclusively for recreational sport-divers to assess the frequency of injured lobsters. We also conducted (1) a tethering experiment to assess the influence of disturbance and injury on predation-induced mortality, and (2) Y-maze laboratory experiments to assess the effect of injury and disturbance on the shelter choice of conspecifics. Injury surveys revealed an immediate increase (to 27.16%) in the percentage of injured lobsters from pre- to post mini-season within patch reef habitats, but only a small increase in injured lobsters amongst patch head habitats. Tethering trials revealed that injured lobsters were more likely to succumb to predation than uninjured controls, while the shelter choice experiments demonstrated that the usual ability to attract conspecifics was altered by injury. Considering the ability of this fishery to unintentionally injure >25% of the lobster population in patch reef habitats in just 2 d, the potential population consequences to spiny lobsters must be examined. KEY WORDS: Indirect behavioral disturbance · Sublethal injury · Unobserved mortality · Predation · Limb damage · Recreational fishing · Panulirus argus · Caribbean spiny lobster · Florida Keys Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 303. Online publication date: November 21, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Laboratory evidence for behavioural impairment of fish escaping trawls: a review

Abstract It is now widely accepted that for some species a proportion of the undersized fish escaping trawl codends die as a direct result of stress, with 10% to 30% mortality commonly cited. It has also been suggested that there may be indirect or behaviourally mediated mortality; fish that encounter and escape the trawl, only to experience stress-induced behavioural deficits and succumb to predators in the hours or days afterwards. The goal of this review was to evaluate the plausibility of this behaviourally mediated, yet unobserved mortality. Three laboratory studies utilizing cod (Gadus morhua), walleye pollock (Theragra chalcogramma), and sablefish (Anoplopoma fimbria) have assayed for behavioural impairment in fish following application of stressors designed to simulate entrainment and escape from trawls. Where impairments in anti-predator capabilities occurred, it was determined that trawl-stressed fish exhibited reduced swimming speed, reduced shoal cohesion, and reduced predator vigilance compared to control fish. Although stressed fish appeared to rapidly recover their ability to avoid being eaten by predators, measurements of more subtle aspects of escapee behaviour suggest that impairments may persist for days after stressor application. Although these studies demonstrate that more investigation is required, when combined with a more extensive literature demonstrating that a variety of stressors can impair fish anti-predator behaviour, it is reasonable to conclude that many fish species escaping trawl codends will likely suffer behavioural deficits that subject them to elevated predation risk. As such, there is probably mortality associated with trawl fisheries that is generally unrecognized, unmeasured, and unaccounted for in current stock assessment models. Further, these studies demonstrate that behavioural competency needs to be considered in the design and implementation of by-catch reduction devises and strategies.

Behavioral impairment after escape from trawl codends may not be limited to fragile fish species

Field studies indicate great variability between fish species in their susceptibility to direct mortality resulting from stress incurred during entrainment and escape through trawl codends. Moreover, stressors that do not directly kill fish may still cause indirect mortality, such as behavioral impairment leading to predation. However, it is unknown whether resistance to direct mortality also imparts resistance to behavioral impairment. Juvenile sablefish Anoplopoma fimbria are more resistant to direct mortality resulting from physical damage and stress than are walleye pollock Theragra chalcogramma. We measured juvenile sablefish resistance to behavioral impairment resulting from simulated trawl passage and compared results to those for walleye pollock, a species already studied. Juvenile sablefish (18–27 cm) were subjected to three levels of simulated trawl/escape stressors in the laboratory: (1) control: no stressors; (2) swim: forced swimming for 90 min at 0.33 m s −1 in a towed net, followed by escape through 8 cm square mesh; and (3) swim/pin: forced swimming for 60 min, then pinning against net meshes for 30 min, followed by 3 min crowding prior to escape. Subsequently, we examined sablefish behavior in the presence of a threatening but non-lethal predator (38–49 cm sablefish). In a second experiment, equal numbers of trawl-stressed and control fish were mixed and exposed to predation by a lingcod Ophiodon elongatus (62–87 cm). The first experiment demonstrated that sablefish suffered the same behavioral impairments as walleye pollock: stressed fish swam slower, shoaled less cohesively and allowed the predator to approach closer than did controls. In the second experiment all three levels of trawl stress caused fish to be consumed in greater numbers (by lingcod) than control fish, again, like walleye pollock. Therefore, although differing in susceptibility to potentially lethal stressors, both species exhibited similar impairments in response to sub-lethal stressors. This suggests that for numerous fish species, behaviorally impaired individuals escaping codends may be consumed by predators, contributing to unobserved mortality.

Discard analysis and damage to non-target species in the "rapido" trawl fishery

This paper describes the catch composition in the rapido trawl fishery and the direct effects on non-target species. All data were collected on commercial fishing vessels so as to reflect commercial rapido-trawling practice. The effects on non-target species were measured using two different damage scales (three- and seven-level scales) depending on the morphology of the taxa. Damage assessment was performed taking into account the whole fishing process by collecting individuals that passed through the cod-end, individuals that were retained in the cod-end and dropped onto the deck and individuals that were collected at the end of the sorting operation just before their return to the sea. Due to differences in the habitat and spatial distribution of target species, discard/commercial ratio was very different among the three different target species fisheries: 1:6 in the queen scallop (Aequipecten opercularis) fishery, 2:1 in the flatfish (Solea spp., Platichthys flesus, Psetta maximus and Scophthalmus rhombus) fishery and 9:1 in the scallop (Pecten jacobaeus) fishery. Damage sustained by non-target species was species-specific and related to the morphology of different organisms. The sorting operation produced similar levels of injury to those of the gear itself: all discarded animals showed higher levels of damage after the sorting than before. Damage to animals that had passed through the cod-end followed the same pattern, and these data could give an estimate of the "unobserved mortality". Our observations indicated a higher impact on non-target species caused by the queen scallop fishery than that caused by the flatfish fishery. This could be due to the total amount of hard-shelled species (in the queen scallop fishery, A. opercularis accounted for 87% of the total catch biomass) in any given haul, since shells macerated the catch during towing. Discarded animals from the queen scallop fishery showed higher levels of damage than those collected in the flatfish fishery. The rapido trawl fishery seemed to exert a strong selective pressure on the macrobenthic community, being able to modify the epibenthic fauna structure which, in heavily exploited fishing grounds, was dominated by bivalves, gastropods, crabs, starfish and brittlestars.

Trap-induced mortality of the spiny lobster, Panulirus argus, in Florida, USA

In Florida’s lobster fishery, sublegal-sized spiny lobsters, Panulirus argus, are commonly placed in traps to attract legal-sized lobsters. Many died from exposure to air during transport before the use of live wells and still die because of confinement in traps. Much of this mortality is not apparent during normal trap-fishing operations, and the magnitude of the unobserved mortality remains the subject of controversy between fishermen and fishery managers. After fishermen began using live wells in the 1987–88 fishing season, the harvest of legalsized lobsters increased. Initially, the increase was smaller than predicted, apparently because the average number of traps in the fishery increased from 576 000 during the 1977–78 to 1986–87 fishing seasons to 854 000 during the 1987–88 to 1992–93 seasons. High numbers of traps in the fishery have been implicated as contributing to increased mortality of sublegal-sized lobsters. When the average number of traps was reduced to 605 000, after the 1993–94 season, the harvest predictions attributed to live-well use were largely achieved. Observations on commercial fishing vessels were used to reevaluate previous harvest predictions and develop additional coefficients for fishing mortality related to exposure and confinement in traps.

The Analysis and Interpretation of Seedling Recruitment Curves

We present statistical methods appropriate for the analysis of seedling recruitment data. The techniques are applied to the data collected by H. A. Roberts and his colleagues. The main result of this analysis is that seed banks do not in general decay according to a negative exponential pattern and that a wide range of recruitment patterns occur. We explore the effects of unobserved mortality on age-specific recruitment. We also demonstrate the difficulty of interpreting age-specific patterns of recruitment in terms of the behavior of individual seeds. This difficulty is a result of unobserved seed mortality and between-seed variability, either of which can qualitatively change the shape of the recruitment curve. A simple model of the recruitment process is presented, and it is demonstrated that density dependence can generate spurious patterns of age-specific recruitment.