The effect of tidal height and stocking density on growth of Atrina maura Sowerby, 1835, cultured in bottom plots

By Francisco Cordoza-Velasco

La Paz, Baja California Sur, Mexico.

Email: fcardoza@cibnor.mx

Penshells are bivalve molluscs belonging to the Pinnidae family. They are widely distributed in tropical and subtropical environments (only a few temperate species are reported) and have been used for different purposes by many different cultures around the world.

In Mexico, all the members of this family are known as 'hashas' (hatchet) and are considered a highly prized food item. Their posterior abductor mussel is widely marketed domestically and sometimes the much smaller anterior abductor muscle and even the mantle are also consumed. At this time, penshell meat may reach a price as high as 150 Mexican pesos/Kg (9.40 US$/pound).

The improvement in fishing gear efficiency has had a very strong impact on wild penshell populations. Production trends have been steadily declining over the past ten years and no efforts have been made either to assess their actual status or to impose regulations to fishing activities. In order to find an alternative mode of production, the Centro de Investigaciones Biológicas del Noroeste (CIBNOR) in La Paz, Mexico, has started a research programme aimed at designing a culture technology for Atrina maura, one of the two most common penshell species in the Oriental Pacific Ocean.

Penshell aquaculture has been successfully practiced in countries such as China, Korea, and Hong Kong. In Mexico, culture trials have been performed on Pinna rugosa Sowerby, 1835, yielding encouraging results. However, Atrina maura is preferred by local fishermen because of its higher price and post-harvest handling advantages.

As part of this project, the effects of tidal height and stocking density on shell growth and abductor muscle weight were evaluated. 2500 penshells were transplanted from suspended culture to bottom plots at Rancho Bueno, a coastal lagoon in the Mexican Pacific. These plots, located in two different tidal zones, were surrounded by steel rebar fences covered with plastic mesh and contained four different stocking densities.

After fifteen months, anteroposterior shell length and abductor muscle wet weight were measured, and data were statistically processed to find out if there was a difference between the means for each treatment. Results could be interpreted from two different points of view:

1. Biological There undoubtedly was a significant effect of both tidal height and stocking density on muscle weight. Shell length, however, was significantly influenced by tidal height, but not by stocking density.

Although statistical tests showed that muscle weight was significantly higher at lower stocking densities, we are speaking about average individuals whose muscle is a few grams bigger than those raised under crowded conditions.

2. Bioeconomic Although individual muscle weights could be a few grams smaller under crowded conditions, the number of animals planted in that area could result in a higher Production per Area Unit (PAU).

Muscle weight data from each sample were summed and extrapolated to obtain a proportional PAU (assuming different mortality values). Based on these speculations, we find it very improbable that PAU at higher stocking densities (e.g. 75 individuals / m2) could be lower than that in lower stocking densities (15 individuals / m2), even if the mortality in the former is as high as 75%.

We have reached the conclusion that, since there is not a difference in price due to muscle size (as in pectinids), a higher stocking density must be preferred by aquaculturists. On the other hand, an intertidal site does not offer more advantages than a subtidal site, thus a permanently submerged location must be preferred.

I would like to thank the Malacological Society of London for their invaluable support, via the Centenary Research Grant, in making this research possible. Special thanks also to Mr Albert Brandt for his help in editing the original manuscript.