The mysid Neomysis mercedis was examined as a test organism for use in acute toxicity tests at intermediate salinities characteristic of estuarine waters. Several sensitive invertebrate species are available for marine assessments (mysids) and freshwater tests (cladocerans), but few are available for estuarine toxicity tests. Observations in the laboratory indicate that Neomysis mercedis can be reared successfully at a temperature of 17°C, a salinity of 2‰, and a population density less than 5/L. Brine shrimp nauplii Artemia salina, algae, and commercial foods were used to sustain mysid cultures. Neomysis mercedis is viviparous and can complete its life cycle in 3–4 months. Neomysis mercedis is as sensitive as or more sensitive to toxicants than the marine mysid Mysidopsis bahia and the freshwater cladocerans Daphnia magna, Ceriodaphnia dubia, and Simocephalus serrulatus. The mean 96‐h LC50 values (concentrations lethal to half the test animals) for N. mercedis, in increasing order, were 0.20 μg/L for methyl parathion, 2.2 μg/L for malathion, 14 μg/L for carbofuran, 150 μg/L for copper sulfate, 280 μg/L for thiobencarb, and 1,600 μg/L for molinate. Neonates (5 d postrelease) were generally more sensitive than older juveniles. Coefficients of variation (100·SD/mean) of LC50 values varied from 21 to 35%.
The cause of toxicity to young striped bass Morone saxatilis and mysids Neomysis mercedis from an agricultural drain (Colusa Basin Drain, Sacramento River Valley, CA) containing tail water from rice fields was investigated. Previous studies had postulated that toxicity was due to one or more of the insecticides carbofuran, malathion, and methyl parathion used on rice in California. The discharge of insecticide‐tainted agricultural drain water into the Sacramento River may have contributed to the decline of the striped bass population in the Sacramento‐San Joaquin Estuary, either directly by mortality or indirectly by reducing food items such as mysids. Trace metals cadmium, copper, and zinc from abandoned mining operations may also have contributed toxicity. Water samples were collected from the Colusa Basin Drain and upstream and downstream from the Sacramento River, analyzed for insecticides and trace metals, and tested for toxicity. No significant toxicity to striped bass was observed for either water body, and no significant toxicity to Neomysis mercedis was observed for the Sacramento River sites. However, Colusa Basin Drain water was significantly toxic to Neomysis mercedis, which was attributed to lethal (up to three times the 96‐h LC50 value) concentrations of methyl parathion. The trace metals, carbofuran, and malathion were present at less than acutely toxic concentrations. The addition of 2 ppt salinity appeared to increase survival of both test organisms. This study clearly demonstrates the need for reducing environmental levels of methyl parathion.
The Delta Smelt Hypomesus transpacificus is a protected osmerid endemic to the San Francisco estuary of California. We conducted laboratory tests on marked versus unmarked juvenile adult smelt to evaluate (1) calcein mark intensity and postmarking survival for juveniles and adults, (2) photonic mark retention and survival of adults, and (3) predation by juvenile Striped Bass Morone saxatilis. Calcein mark intensity was graded in six body sections and adults were photonically marked using four fin‐color combinations. Across all immersion times (1–7 min) all fish showed 100% mark retention 7 d after exposure to calcein concentrations of 2.5 and 5.0 g/L of water. Average survival 7 d after calcein marking was 93.9% in juveniles and 98.6% in adults. After 97 d of calcein and photonic marking, adults had weaker double marking, but each type of mark still showed 100% retention. Average survival of adult fish 70 d after marking was 98.7%. Unmarked and calcein + photonically marked adult Delta Smelt exposed to juvenile Striped Bass did not experience significantly different predation rates. Calcein is both effective and practical to batch‐mark juvenile and adult Delta Smelt. Combined calcein and photonic marking for adult Delta Smelt further enables identification of multiple groups while potentially improving mark detection in short‐term studies.Received October 1, 2012; accepted August 22, 2013Published online January 14, 2014
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