胃内容情報を用いたマエソ属の摂食戦略についての2・3の試論

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  • What insights can be gain from stomach contents ? : Essay on the feeding strategy of the Lizard fish from informations gathered from the stomach contents
  • イ ナイヨウ ジョウホウ オ モチイタ マエソゾク ノ セッショク センリャク

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1) Introduction : Stomach contents are a first hand clue to the feeding activities of fishes. However, due to the complexity of biotic phenomena it is very difficult to reach a clear understanding of their relationship to feeding strategies. Here, some trails were made using various perspectives based on the stomach contents of the Lizard fish. 2) Material and Method : Prey are often found in varying degrees of digestion within the stomach. In order to estimate the food intake of the fish, it is thus necessary to reconstruct to the original the weight of each prey item. Thus, ration size(r), the summation of the weights of individual prey found in the stomach, restored to their original size based on length/weight and/or vertebral column segment size/body length relationships5)7), was deemed a more accurate indicator of feeding than the unaltered stomach content weight, (f). The Lizard fish (Saurida elongata and S. undosquamis) were selected for analysis from 12 systematic experimental trawls conducted throughout the eastern Seto Inland Sea of Japan, Tab.2-1. See Notation & Abbr.,Tab. 2-2. 3) On Ration, Prey Size and Satiation in the Field : For the predator, the most favorable circumstances naturally occur when prey resources are abundant during a feeding period (Cruise 8). The quantity of prey ingested by the Lizard fishes ranged widely though composed mainly of one to several Anchovies, Fig. 3-2, while the number of prey ingested varied according to the size of individual prey. That is, an inverse relationship existed between the number of prey ingested and prey size with fewer prey being ingested when prey were larger, Fig.3-3. Also, the interval between feedings for predators grouped according to the number of prey items found in the stomach indicates that the feeding intesvals varied with prey number, Fig.3-4. In addition Fig.3-5 the size of the last ration was estimated from the relative stomach contents just prior to (f) and the amounts of food ingested during the last feeding bout (F). Combined with data on the physiological maximum of these species, it was found that the Lizard fish, while posessing the capacity to feed at levels up to 40% of its body weight, does not normally do so in the field, Fig.3-1, and once satiated, subsequent prey were not ingested until the stomach contents are reduced by digestion to roughly half of the satiation level. Thus, there appears to be a cessation of feeding by fishes in the field when ecological satiation is achieved. 4) On the Probability of Prey Ingestion in the Natural Environment : In the course of time after the ingestion of a prey item the stomach contents change in the following manner : P (Ingested prey identifable) - A (Amorphous contents) - E (Empty stornach), Fig. 4-1. From this, one schematical model of feeding activities at the group level, using P, A and E as criteria, was devised, Fig. 4-3A. From intital analysis based on the premise that prey are distributed at random in space, possible compositions of P, A and E were found located on the lines defined by the points a-b-c-d-e, Fig. 4-3B. These points correspond to those of Fig.4-3A. Looking at the data from serveral cruises, one can see that this assumption is very seldom supported by actual field data, Fig.4-2. Subsequently, the more probable assumption that prey distribute in patches in spaces implies a negitive binomimal distribution: (q-p)¯-k After simulations using variable parameters of m and P, smaller k values, which imply that prey are distributed with increasing patchiness, are located nearer to E between the A-E axis for any level of P. The level of P in turn, implies the abundance of prey resoures, Fig.4-4, Fig.4-5. Thus, we can read from the informations concerning P, A and E that the chances of ingestion of prey by any group of predators in the field comes not only from the abundance of prey resources but also from the usually patchy spatial distribution of the prey. 5) On the Cost of Prey Inges

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