MACROALGAE GROWTH: ASSESSMENT OF EXISTING MODELS AND LIMITATIONS

John Colt
 
NOAA Fisheries
Northwest Fisheries Science Center
2725 Montlake Blvd E.
Seattle, WA 98112
email: john.colt@noaa.gov
 

There is a lack of uniformity in the literature regarding the names used for these models; care must be taken to avoid confusion. This presentation will focus only on macroalgae growth in land-based intensive tumble culture.

Most of the reported macroalgae growth rates are only based on final and initial weights. With only two data points, all of these equations give a perfect fit to the data but tell nothing about the accuracy of the models for other times or conditions The standard deviations of the predicted values for these three models were estimated from the growth data for Kappaphycus spp. presented by Yong et al. (2013). Based on the average growth parameters over the entire 7-day experiment, the linear growth model had lower standard deviations than for the specific growth rate (SGR) or daily growth rate (DGR). The fits based only on the initial and final weights were better than for average growth rates approach. For LIN and SGR, the best fit was produced by growth rates based on linear regression.

Before getting caught up in the details of fitting equations to data, it is best to step back and examine the characteristics of macroalgae growth in intensive tumble culture. When adequate nutrients and inorganic carbon was provided, Demetropoulos & Langdon (2004) found that the specific growth rate of Pacific dulse was a strong function of macroalgae density. An increase in density from 2 to 6 kg/m2 resulted in a reduction of the SGR by a factor of 2.8. As a result, growth rate and yields, depend strongly on initital densities and harvest intervals. Comparison of growth data based on different initial densities and harvest intervals is misleading.

It is unlikely that a single conventional growth model will be valid over a wide range of densities or harvest intervals. At low densities, the specific growth rate or daily growth rate may be a good fit. At moderate density where the growth rate starts to drop off, the linear grow model may be a better fit. At high densities where growth rate and yields drop, it is unlikely if any of the conventional models will be valid.

Density-dependent growth is commonly observed in a number of different plant populations. Competition within a population is termed self-thinning. It may be possible to develop a self-thinning model that covers the full range of commercial macroalgae densities.