Migratory environmental history of the grey mullet Mugil cephalus as revealed by otolith Sr : Ca ratios

We used an electron probe microanalyzer (EPMA) to determine the migratory environmental history of the catadromous grey mullet Mugil cephalus from the Sr:Ca ratios in otoliths of 10 newly recruited juveniles collected from estuaries and 30 adults collected from estuaries, nearshore (coastal waters and bay) and offshore, in the adjacent waters off Taiwan. Mean (+/-SD) Sr:Ca ratios at the edges of adult otoliths increased significantly from 6.5 +/- 0.9 x 10(-3) in estuaries and nearshore waters to 8.9 +/- 1.4 x 10(-3) in offshore waters (p < 0.01), corresponding to increasing ambient salinity from estuaries and nearshore to offshore waters. The mean Sr:Ca ratios decreased significantly from the core (11.2 +/- 1.2 x 10(-3)) to the otolith edge (6.2 +/- 1.4 x 10(-3)) in juvenile otoliths (p < 0.001). The mullet generally spawned offshore and recruited to the estuary at the juvenile stage; therefore, these data support the use of Sr:Ca ratios in otoliths to reconstruct the past salinity history of the mullet. A life-history scan of the otolith Sr:Ca ratios indicated that the migratory environmental history of the mullet beyond the juvenile stage consists of 2 types. In Type 1 mullet, Sr:Ca ratios range between 4.0 x 10(-3) and 13.9 x 10(-3), indicating that they migrated between estuary and offshore waters but rarely entered the freshwater habitat. In Type 2 mullet, the Sr:Ca ratios decreased to a minimum value of 0.4 x 10(-3), indicating that the mullet migrated to a freshwater habitat. Most mullet beyond the juvenile stage migrated from estuary to offshore waters, but a few mullet less than 2 yr old may have migrated into a freshwater habitat. Most mullet collected nearshore and offshore were of Type 1, while those collected from the estuaries were a mixture of Types 1 and 2. The mullet spawning stock consisted mainly of Type 1 fish. The growth rates of the mullet were similar for Types 1 and 2. The migratory patterns of the mullet were more divergent than indicated by previous reports of their catadromous behavior.