Download Productive fishery and decreased risk of over

Use of bomb radiocarbon to validate age in the North Atlantic population of Polyprion americanus,
including re-calculation of natural mortality estimates from life history parameters
Introduction
Wreckfish, Polyprion americanus, is a commercially important, long-lived demersal fish,
occurring throughout the Atlantic. Wreckfish can be found in the eastern Atlantic from
Norway to South Africa, including the Mediterranean, Canary Islands, Azores, Bermuda, and
Madeira. Along the western North Atlantic they can be found from Grand Banks,
Newfoundland to La Plata River, Argentina (Ball 2001 and Sedberry 1999). Adult wreckfish can
be found at depths of 40-800 m; however, most occur in waters deeper than 300 m, with a
maximum reported depth of 1000 m (Sedberry 1999). At these depths adult wreckfish
concentrate around steep, rocky bottoms and deep coral reefs, but can be found in lower
concentrations along flat hard bottom (Sedberry 1999). Despite the wide geographic range,
recent DNA evidence distinguishes a North Atlantic and South Atlantic population (Ball 2000).
This study will focus on the North Atlantic population of P. americanus.
Commercial fishing for wreckfish takes place throughout its North Atlantic range, with
the exception of Bermuda. The western North Atlantic fishery is considered a success story of
fisheries management, but important life history information is still lacking. For instance,
much is unknown about the early life history of juvenile North Atlantic wreckfish and although
age estimates have been published, no age validation study has been performed. The latter
may result in inaccurate estimates of various age related biological parameters. The current
study will use bomb radiocarbon analysis to validate current age estimates for this reported
long lived species. Once the ages have been validated, we will estimate natural mortality
rates for possible use in stock assessments and calculations of annual catch limits. In addition,
growth rate back-calculations will be used to characterize early life history traits. Finally,
accurate growth curves will be formulated based on the new age information.
Adam Lytton
South Carolina Department of Natural Resources and College of Charleston
Methods
• Samples used in this study have been previously collected from commercial catches by federal and state port
samplers for the years of 1989-present.
Bomb radiocarbon analysis
• During the 1950s to 1960s radiocarbon surface 14C activity doubled in the world’s oceans, due to extensive
nuclear arms testing (Broecker 1985), ultimately creating a chronometer.
• The core of an otolith, representing the first year of life, can be analyzed for 14C levels.
• Levels can then be plotted and checked for phase agreement.
Back calculations
• A technique that uses a set of measurements made on a fish at one time to infer length at
an earlier time or times.
• These calculations help make up for the lack of smaller specimens and allow for better
estimations of von Bertalanffy growth model parameters, back calculations will be
performed using the biological intercept algorithm:
La=Lc+ (Oa-Oc)(Lc-Lo)(Oc-Oo)-1
Where,
La is the size of a fish at a certain age
Oa is otolith size at a certain age
Lo and Oo are the size of the fish and otolith at the biological intercept
Oc and Lc are the size of the fish and otolith at capture
Fig. 3. An example of a wreckfish otolith core located within the highlighted circle.
Fig. 1. Wreckfish, Polyprion americanus
Over-aged fish
Why Age Validation?
Fig. 5. Conceptual model depicting the calculation of the biological intercept (circled). This model allows for
correction of variation in growth rate amongst a population of fish (Campana 1990).
Natural mortality
• Otoliths of long-lived fishes are notoriously difficult to read with increasing age, because
increment patterns become more compact and harder to differentiate.
• Natural mortality rates will be estimated using the same methodologies as those
employed by the South Atlantic Fishery Management Council.
• There is a large age gap between the reported max ages of the North Atlantic (39 years) and
South Atlantic population (76 years). In addition, P. oxygeneios a congeneric species, is
found to reach ages approaching 60 years.
Log10M=0.0066-0.279 Log10 L∞ + 0.6543 Log10K + 0.4634log10T
Where,
L∞ (cm) and K (yr-1) are estimates from the von Bertalanffy growth model
T is the average temperature of the Blake Plateau
• The North Atlantic population is admittedly thought to have been under aged.
• Accurate age estimates allow for calculation of growth rates, age at maturity, age at
recruitment, as well as natural mortality. Furthermore, these biological parameters can be
analyzed for changes over the years in order to determine the health of a fishery.
Implications
Fig. 4. 14C levels found in haddock exhibiting good phase agreement (Kalish 1993). Fish that may have been over–
aged would fall to the far left of the curve, while those under-aged would fall to the right of the curve.
Life history calculations
• Growth curves will be determined using the von Bertalanffy growth model:
Due to under aging, natural mortality rates and life history traits used in previous stock assessments
may have been incorrect. This project proposes to validate ages and aging techniques, allowing for
accurate estimation of life history traits. This information can then be used by managers to properly
manage the species, as well as maximizing a healthy economic return from the fishery.
Lt = L∞(1 – e−K(t −t ))
0
Where,
Lt is the expected or average length at time (or age) t,
L∞ is the asymptotic average length
K is the von Bertalanffy growth rate coefficient
t0 is a modeling artifact that represents the time or age when the average length was zero.
Proximal
Acknowledgements and Funding:
Marine Resources Monitoring, Assessment, and Prediction Program (MARMAP)
of SCDNR
National Marine Fisheries Service
Marcel Reichert(MARMAP)
Joey Ballenger (MARMAP)
Tracey Smart (MARMAP)
Literature Cited
Ball, A. O., Sedberry, G. R., Zatcoff, M. S., Chapman, R. W. and Carlin, J. L. 2001.
Population structure of the Wreckfish Polyprion americanus Determined with
Microsatellite Genetic Markers. Mar. Bio. 137:1077-1090.
• There is a well defined generalized relationship between K and L∞. In addition, parameters can be
used in estimation of natural mortality and allow the health of a population to be monitored over
time.
Dorsal
Ventral
Distal
Fig. 2. Transverse sagittal otolith section exhibiting proximal increment compaction. Each red dot
represents one year, with this fish estimated to be 46 years of age.
L∞
and
K =
Productive fishery and
decreased risk of overharvesting
Broecker, W. S., Peng, T. H., Ostlund, G. and Stuiver, M. 1985. The Distribution of
Bomb Radiocarbon in the Ocean. J. Geophys. Res. 90:6953-6970.
Campana, S. E. 1990. How Reliable are Growth Back-Calculations Based on
Otoliths? Can. J. Fish. Aquat. Sci. 47:2219-2227.
Kalish, J. M. 1993. Pre- and Post-Bomb Radiocarbon in Fish Otoliths. Earth Planet.
Sci. Lett. 114:549-554.
Photo courtesy: swordfishingcentral.com
Sedberry, G. R., Andrade, C. A., Carlin, J. L., Chapman, R. W., Luckhurst, B. E.,
Manooch, C. S., et al. 1999. Wreckfish Polyprion americanus in the North Atlantic
Fisheries, Biology, and Management of a Widely Distributed and Long-Lived Fish.
Amer. Fish. Soc. Symp. 23:27-50.