Selective predators or choosy fishermen? Relation between fish harvest, prey availability and great cormorant (Phalacrocorax carbo sinensis) diet

Figures & data

Figure 1. Map of the study area. The two study sites: A, Lake Como and B, lakes of Varese basin (corresponding to Lake Varese, Lake Comabbio and Lake Monate) are enclosed by dotted lines. Location of roosts in the study area is indicated by dots, and those chosen to collect samples by triangles: Brabbia roost and Faggeto roost.

Table I. Monthly numbers of wintering cormorants counted at seven roosts regularly monitored during the 3 years of the study, and montly average cormorant days in the whole study area.

Month	2000/2001	2001/2002	2002/2003	Cormorant days
September	0	223	0	2230
October	229	134	453	8432
November	221	1087	300	16080
December	918	1060	1144	32261
January	953	811	785	26340
February	1275	864	671	26227
March	629	912	719	23353
April	10	67	4	810

Table II. Numerical abundance (number and percentage of fish for each taxonomic category) and biomass (g) of various fish taxa in relation to roost site.

Taxonomic categories	Brabbia roost (Varese)			Faggeto roost (Como)
	N	%	Mass (g)	N	%	Mass (g)
Alosa fallax				2	0.4	388
Coregonus sp.	2	0.5	148	2	0.4	372
Exos lucius	1	0.2	6
Rutilus sp.	5	1.1	257	17	3.3	252
Leuciscus cephalus cabeda	1	0.2	145	11	2.1	1802
Alburnus alburnus alborella	3	0.7	6	42	8.2	169
Tinca tinca				3	0.6	603
Scardinius erythrophthalmus	169	38.3	36635	23	4.5	5688
Carassius carassius	4	0.9	636
Undetermined cyprinids	110	24.9	651	292	56.9	1255
Ictalurus melas	70	15.9	2473	2	0.4	132
Lota lota				45	8.8	6946
Lepomis gibbosus	3	0.7	146	13	2.5	321
Perca fluviatilis	58	13.2	2441	45	8.8	1726
Stizostedion lucioperca	3	0.7	307
Undetermined Perciformes	8	1.8	539	3	0.6	202
Undetermined fish	4	0.9	22	13	2.5	74
Total	441	100	44414	513	100	19932

Figure 2. Reconstructed fish biomass of consumed prey (% of total biomass) in each study site (Como study site corresponding to Lake Como, pellets collected from Faggeto roost; Varese study site corresponding to Lake Varese, Lake Comabbio and Lake Monate, pellets collected from Brabbia roost).

Table III. Cormorant diet data (biomass of fish species consumed by cormorants) compared with annual harvesting of professional fisheries (biomass fish species harvested/ha) in the two study areas (Como, Varese).

Species	Cormorant diet (consumed biomass, g)	Cormorant diet (%)	Commercial fishing (kg harvested)	Commercial fishing (%)
Study area: Como
Coregonus sp.	373.89	1.88	22.35	62.83
Lepomis gibbosus	354.05	1.78	0.00	0.00
Rutilus sp.	290.00	1.45	0.22	0.61
Leuciscus cephalus cabeda	2075.01	10.41	2.05	5.77
Scardinius erythrophthalmus	6550.74	32.87	0.00	0.00
Alburnus alburnus alborella	195.44	0.98	0.03	0.07
Tinca tinca	694.82	3.49	0.41	1.16
Alosa fallax	389.14	1.95	5.57	15.66
Esox lucius	0.00	0.00	0.23	0.65
Lota lota	6972.39	34.98	1.46	4.10
Ictalurus melas	132.39	0.66	0.00	0.00
Perca fluviatilis	1903.73	9.55	3.25	9.13
Study area: Varese
Coregonus sp.	148.48	0.33	4.28	17.00
Lepomis gibbosus	173.32	0.39	0.00	0.00
Rutilus sp.	261.27	0.59	0.00	0.00
Leuciscus cephalus cabeda	147.48	0.33	0.00	0.00
Scardinius erythrophthalmus	37288.11	83.95	5.09	20.22
Alburnus alburnus alborella	6.00	0.01	0.00	0.00
Carassius carassius	647.42	1.46	12.83	50.92
Cyprinus carpio	0.00	0.00	0.13	0.51
Tinca tinca	0.00	0.00	0.52	2.05
Esox lucius	5.70	0.01	0.21	0.81
Ictalurus melas	2473.97	5.57016	0.82	3.24
Perca fluviatilis	2897.90	6.524644	0.54	2.16
Stizostedion lucioperca	365.04	0.821895	0.14	0.56
Silurus glanis	0.00	0	0.64	2.53

Figure 3. Comparison between results of pellet analysis from Brabbia roost (cormorant diet) and data from haul sampling on the lakes of Varese district (Lake Varese, Lake Comabbio and Lake Monate) as an estimator of fish availability.

Figure 4. Proportion of fish collected from nets (index netting) and fish consumed by cormorants (cormorant diet) for each of the five identified size classes.