Estimating daily ration of skipjack tuna on larval and juvenile anchovy in the Kuroshio Oyashio transition region in early summer

Transcription

1 Estimating daily ration of skipjack tuna on larval and juvenile anchovy in the Kuroshio Oyashio transition region in early summer Sayaka Nakatsuka 1, Akinori Takasuka 2, Hiroshi Kubota 2 and Yoshioki Oozeki 1, 2 1 Tokyo University of Marine Science and Technology 2 National Research Institute of Fisheries Science, Fisheries Research Agency 1

2 2 Introduction Catch(10 5 t) Sardine Mackerel 60 Jack mackerel Anchovy6 50 Sardine Mackerel 10 spp Year Dominance of small pelagic fish has shifted at decadal scales. High mortality during early life stages is a regulator of population dynamics. Predation is a major source of mortality throughout the life and thus a determinant of recruitment. Estimating daily ration of predators feeding on larvae and juveniles is required for quantifying recruitment. Jack mackerel Anchovy

3 3 Introduction Feeding habits of predators have often been reported, but a few studies analyzed prey predator interactions in the field. Distribution of larval and juvenile small pelagic fish and predators (Takahashi et al. 2001) Growth selective predation on larval anchovy (Takasuka et al. 2003, 2004) Qualitative aspects were well been reported, however quantitative data have not well obtained. => Estimating daily ration of predators feeding on larvae and juveniles are essential for predicting recruitment.

4 4 Introduction Skipjack tuna was one of predatory species of larvae and juveniles. Culture experiment Culture experiment The daily ration of skipjack tuna was estimated as 15 % of body weight (Magnuson, 1969) => Probably overestimated

5 5 Objectives Quantitative estimation Daily ration of predators feeding on larvae and juveniles Daily cycle of feeding of skipjack tuna Daily ration of skipjack tuna feeding on larval and juvenile anchovy

6 Materials and methods Drift May June, GPS 2006 Kuroshio Oyashio buoy transition region Sampling areas and stations for skipjack tuna GPS Buoy Drift gill net 50 m 10 m 1 net panel 30 net panels Drift gill net Net panels: 30 net panels Mesh size: mm Net setting: hours 6

7 Materials and methods May June, 2006 Kuroshio Oyashio transition region Sampling areas and stations for skipjack tuna GPS Buoy Drift gill net 50 m 10 m 1 net panel 30 net panels Drift gill net Net panels: 30 net panels Mesh size: mm Net setting: hours 7

8 8 Gut contents analysis The digestive tract was divided into 4 portions by the anatomical method. (Tominaga and Shibusawa 1978) Stomach Intestine I Intestine II Intestine III Stomach Intestine II Stomach contents Undigested larval anchovy Intestine I Intestine III Otolith

9 Estimating standard length and body weight of anchovy Maximum radius measured for the otoliths from digestive tract Standard length (mm) Relationships between otolith radius and standard length y = 0.826x r 2 =0.934 n= Otolith radius (µm) Wet weight (g) Relationships between standard length 5 and body weight 4 y= x r 2 = Standard length (mm) 9

10 24 hours sampling Drift gill net Net panels: 30 net panels Net setting: hours N=352 The number of the skipjack tuna captured increased at night A total of 352 individuals of skipjack tuna were collected. =>Digestive tracts of 85 individuals were analyzed. FL 47.0 ± 3.5 cm BW 2.1 ± 0.6 kg 10

11 Daily cycle of feeding Otolith of larval and juvenile anchovy (in number) Undigested prey items (in weight) Number of otoliths Skipjack tuna fed during the daytime, and did not feed anymore in the midnight. Start time of feeding: Sunrise (ca. 3:50) End time of feeding: Sunset (ca. 18:30) Weight of undigested prey items (g) 11

12 Estimating daily ration of skipjack tuna Composition of prey items (in number) Composition of prey items (in weight) Daily ration of skipjack tuna on larval and juvenile anchovy Intestine I contents Otolith of larval and juvenile anchovy Daily ration of skipjack tuna on prey items other than larval and juvenile anchovy Stomach contents Undigested prey items Total daily ration of skipjack tuna 12

13 Exponential model of daily ration (Elliott and Persson 1978) C t = F t = ( St S0 e 1 e Rt Rt ) Rt S: the rate of stomach and intestine contents F: the rate of food consumption R: the rate of gastric evacuation C t : the actual amount of food consumed Expression of the rate of gastric evacuation (R) ln (S t ) ln (S ) = 0 Rt ln(average number of otolith) R of otolith of larval and juvenile anchovy Intestine R = 0.32 y = -0.32x r 2 = ln (average weight of undigestedparts ) R of undigested prey items (e.g. fish, crustaceas) Stomach R = 0.63 y = -0.63x r 2=

14 14 Estimating daily ration of skipjack tuna Daily ration of skipjack tuna on larval and juvenile anchovy Daily ration of skipjack tuna on prey items other than larval and juvenile anchovy Feeding weight / body weight (%) 3.6 % Feeding weight / body weight (%) 4.5 % Daily ration of skipjack tuna (avg. 47 cm, 2.1 kg) on larval and juvenile anchovy corresponded to 3.6 % of the body weight.

15 Feeding weight / body weight (%) Daily ration of skipjack tuna on larval and juvenile anchovy 3.6 % Feeding weight / body weight (%) Daily ration of skipjack tuna on prey items other than larval and juvenile anchovy 4.5 % Total daily ration of skipjack tuna Feeding weight / body weight (%) 8.1 % An individual skipjack tuna (avg. 47 cm, 2.1 kg) was estimated to consume ca individuals (79.7 g) of larval and juvenile anchovy per day. This corresponds to 45 % of the total daily ration. 15

16 16 Summary Daily cycle of feeding pattern => Daytime: Feeding => Nighttime: No feeding Daily ration of skipjack tuna was 8.1 % (in weight) Daily ration of skipjack tuna on larval and juveniles anchovy was 3.6 % (in weight) Larval and juvenile anchovy accounted for 45 % of the total daily ration of skipjack tuna

17 17 Potential predators collected by drift gill nets (in number) Neon flying squid Common dolphinfish Other Pacific pomfret Skipjack tuna occupied 61.8 % of potential predators collected around the anchovy shoals. Estimating the daily rations of the other predators by a similar method.