I am currently conducting research on shrimp stock assessment, and I am utilizing the ‘TropFishR’ Package to analyze a dataset containing monthly carapace length frequency data. The accurate calculation of natural mortality is essential for my analysis of the exploitation rate and other critical factors. In the ‘TropFishR’ package, there are several methods available for calculating shrimp mortality, including Alverson and Carney (1975), Hoenig (1983) - Joint Equation, Hoenig (1983) - Fish Equation, Pauly (1980) - Length Equation, Then (2015) – tmax, Then (2015) – growth, and other techniques.
However, the majority of these methods have been previously utilized for fish total length and standard length, which was not problematic. When I applied Then (2015) – growth and Pauly (1980) - Length Equation, two of the most widely used methods for calculating natural mortality, to the carapace length of shrimps, which is 3 to 6 times shorter than the total length, I observed abnormally high natural mortality rates.
To overcome this issue, I calculated the total length of the shrimp using a regression relationship between total length and carapace length, which allowed me to recalculate the natural mortality. Unfortunately, the calculated values still remained high (>2), with the exception of Alverson and Carney (1975) and Hoenig (1983) - Joint Equation methods, which yielded natural mortality rates of approximately 1.7.
I would greatly appreciate any suggestions or recommended articles that may assist me in addressing this issue.
The issue you are facing is not uncommon, as most mortality estimators were developed for fish species and may not be suitable for shrimp or other crustaceans. The difference in body shape and growth patterns between fish and shrimp can result in varying mortality rates.
To address this issue, one approach is to use species-specific data to estimate natural mortality rates. The use of empirical data on mortality rates can provide a more accurate estimate of natural mortality than relying on equations developed for other species or groups.
Another approach is to use a more comprehensive approach to estimating natural mortality, such as the empirical Bayes method, which can account for variability in mortality rates across different life stages and environmental conditions. This method involves combining prior knowledge on mortality rates with available data on size-at-age and fishing mortality to obtain estimates of natural mortality rates that are tailored to the specific species and context of your study.
Additionally, it may be helpful to consult with experts in shrimp stock assessment to determine the best approach for your specific case. A review of the literature on shrimp stock assessment and natural mortality estimation may also provide additional insights and recommendations.
Some articles that may be helpful in addressing this issue include:
- Froese, R. (2006). Cube law, condition factor and weight-length relationships: history, meta-analysis and recommendations. Journal of Applied Ichthyology, 22(4), 241-253.
- Hoenig, J. M. (1983). Empirical use of longevity data to estimate mortality rates. Fishery Bulletin, 82(1), 898-903.
- Martell, S. J., & Froese, R. (2013). A simple method for estimating natural mortality from age composition data. ICES Journal of Marine Science, 70(3), 693-700.
- Pauly, D. (1980). On the interrelationships between natural mortality, growth parameters, and mean environmental temperature in 175 fish stocks. Journal du Conseil, 39(2), 175-192.
- Then, A. Y., Hoenig, J. M., & Hall, N. G. (2015). Generalizing von Bertalanffy growth models: the power function and fractional polynomial approaches. Reviews in Fish Biology and Fisheries, 25(4), 711-730.
Ahmad Al Khraisat My sincere gratitude for taking the time to provide such a helpful and insightful answer to my question regarding the high natural mortality rates in shrimp stock assessment using the ‘TropFishR’ Package and carapace length.
https://www.researchgate.net/post/How_can_I_get_Excel_data_of_any_published_paper_on_crabs_carapace_size-weight_relationship
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