Gaspar R. Poot-López

EFFECT OF RATION AND SIZE HETEROGENEITY ON HARVEST TIME: TILAPIA CULTURE IN YUCATAN, MEXICO

Size heterogeneity is a common phenomenon in aquaculture systems and influences final production and economic yield. Among other factors, this variability is determined by ration. These variables directly affect potential returns and therefore influence the recommendations made for optimum aquaculture system management. As part of the search for more profitable culture strategies, a bioeconomic model was developed to analyze the effect of size heterogeneity and ration on optimum harvest time and size, thus creating a new methodological tool. Size dispersion was included using a continuous, size-structured population model incorporating the effect of ration on growth. Theoretical results were applied in a case study of tilapia culture in Yucatan State, Mexico, in which optimum ration levels and harvest times clearly differed between size heterogeneity and homogeneity models. Case study results indicated the use of different recommended ration and harvest management strategies depending on real and/or potential target market.

Spatial Analysis of the Abundance and Catchability of the Red Octopus Octopus maya (Voss and Solís-Ramírez, 1966) on the Continental Shelf of the Yucatan Peninsula, Mexico

ABSTRACT The red octopus Octopus maya is an endemic species of the continental shelf of the Yucatan Peninsula, Mexico and supports the largest octopus fishery in the American continent. Little is, however, known about its spatial dynamics. The study of the space-time variation of catchability is a key element in the stock assessment, because it allows behavioral aspects of the resource and qualities of the various exploitation strategies to be clarified, which are essential for the fishery management. The objective was to analyze the spatio-temporal variations of the abundance and catchability of the octopus O. maya on the continental shelf of the Yucatan Peninsula, Mexico. During the fishing season of 2012 (August–December), fishery landings from the small-scale fleet in 13 ports of the Yucatan Peninsula, were analyzed to obtain data on catch, effort and fishing zone for O. maya. During the closed season of 2013 (January–July) fishing hauls were performed across a network of stations along the coast of the Yucatan Peninsula. Spatio-temporal variations in mean catch per unit effort (CPUE) in terms of number of organisms per hour of effective fishing were analyzed. Catchability and its variation with size, time, and zone were estimated through a model based on the Leslie transition matrix and frequency distributions of mantle length. The results showed significant changes in the distribution of O. maya. During the fishing season, the greatest abundances were found along the coast of Campeche (western zone), whereas, during the closed season the highest abundances were recorded along the coast of Yucatan (eastern zone). Trends of decreasing and increasing catchability were observed with respect to size according to the different zones. A high catchability was presented for small sizes along the coast of Campeche during the fishing season. The results indicate the need to investigate possible management measures differentiated by region (west-east) to ensure the fishery sustainability.

Producción de tilapia nilótica (Oreochromis niloticus L.) utilizando hojas de chaya (Cnidoscolus chayamansa McVaugh) como sustituto parcial del alimento balanceado

Aquaculture is an alternative for the production of high-quality, low-cost protein, particularly in developing countries with a limited food supply. In rural areas, the availability of alternative inputs is key to improving fish farming production, especially if these inputs are unprocessed. The leaves of tree spinach (Cnidoscolus chayamansa), a bush that grows in Mexico and Central and South America, are one such option. In this work, juvenile tilapia (7-14.5 g) survival, growth rates, and food conversion rates were studied during two seasons (warm and cold), substituting 25 and 50% of the balanced feed rations with raw tree spinach leaves (ad libitum). The experimental design was completely random, with two treatments and one control (100% of the balanced feed ration); three replicates were done in each season. The densities were 36 fish m per replica in the cold season and 44 fish m per replica in the warm season. The weight gain in the treatments with 50 and 75% balanced feed and tree spinach leaves was similar to that of the control group in both seasons. The cold season adversely affected survival, weight gain, and feed conversion rates in all treatments, but the warm season did not. When tree spinach leaves were included in the tilapia diet, the feed conversion rate for the balanced feed was reduced from 9.17 to 33.62% in the cold season and from 15.38 to 40.23% in 835 Producción de tilapia nilótica utilizando hojas de chaya the warm season. The results show that the use of locally available complementary inputs such as tree spinach leaves may favor the development of small-scale tilapia cultures in the tropics.

Nile tilapia production (Oreochromis niloticus L.) using tree spinach leaves (Cnidoscolus chayamansa McVaugh) as a partial substitute for balanced feed

Aquaculture is an alternative for the production of high-quality, low-cost protein, particularly in developing countries with a limited food supply. In rural areas, the availability of alternative inputs is key to improving fish farming production, especially if these inputs are unprocessed. The leaves of tree spinach (Cnidoscolus chayamansa), a bush that grows in Mexico and Central and South America, are one such option. In this work, juvenile tilapia (7-14.5 g) survival, growth rates, and food conversion rates were studied during two seasons (warm and cold), substituting 25 and 50% of the balanced feed rations with raw tree spinach leaves (ad libitum). The experimental design was completely random, with two treatments and one control (100% of the balanced feed ration); three replicates were done in each season. The densities were 36 fish m per replica in the cold season and 44 fish m per replica in the warm season. The weight gain in the treatments with 50 and 75% balanced feed and tree spinach leaves was similar to that of the control group in both seasons. The cold season adversely affected survival, weight gain, and feed conversion rates in all treatments, but the warm season did not. When tree spinach leaves were included in the tilapia diet, the feed conversion rate for the balanced feed was reduced from 9.17 to 33.62% in the cold season and from 15.38 to 40.23% in 835 Producción de tilapia nilótica utilizando hojas de chaya the warm season. The results show that the use of locally available complementary inputs such as tree spinach leaves may favor the development of small-scale tilapia cultures in the tropics.