Page 97 - The Toxicology of Fishes

Toxicokinetics in Fishes 77

A
2000

SURFACE AREA (cm 2 ) 1200 Brook Trout (2-yr-old) Skin 800 µm Rainbow Trout (3-yr old) Skin 1,120 µm Channel Catfish (2-yr old) Skin 1,060 µm
1600

800

400
0
0 200 400 600 800 1000

B
10

8
SURFACE AREA (cm 2 ) 6 Brook Trout (7-day-old) Skin 15 µm Brook Trout (36-day-old) Skin 35 µm Fathead Minnow (7-day-old) Skin 65 µm Medaka (75-day-old) Skin 75 µm Brook Trout (5 mo.-old) Skin 45 µm Brook Trout (6 mo.-old) Skin 65 µm

4

2

0
0.0 0.2 0.4 0.6 0.8 1.0
BODY WEIGHT (g)
FIGURE 3.10 Skin (solid line) and gill (dashed lines) surface areas as a function of body weight. Arrows represent body
weights of different species and life stages and their measured skin thickness (Rodney Johnson, personal communication).
(A) Fish weighing up to 1000 g; (B) fish weighing less than 1 g.

in relative importance in small fish because of the way in which gill and skin surface areas scale to fish
body weight. In smallmouth bass, gill surface area scales to body weight by a fractional exponent of
about 0.78, consistent with the role of the gill in supporting metabolic activity (Price, 1931). Skin surface
area scales instead to a body weight exponent of about 0.67 (Schmidt-Nielson, 1984). Using these values,
Lien and McKim (1993) predicted that skin surface area approaches and may even exceed that of the
gills in fish weighing less than 5 g (Figure 3.10). Reduced skin thickness and increased skin vascular-
ization (Rombough and Moroz, 1990) also contribute to relatively greater dermal uptake in small fish.

Xenobiotic Distribution
Primary Determinants
A variety of processes act to distribute absorbed toxicants within fish. A portion of the absorbed
compound may distribute to a site of action (organ, tissue, fluid) where toxic effects are expressed. Other
sites serve as repositories for the chemical over short or long periods of time, while others are mobile
and provide a means of transit within and out of the animal. During its residence in the body, a toxicant
will redistribute continuously due to changes in chemical concentration gradients that drive xenobiotic
movement.
Five primary factors control the distribution of xenobiotics from blood to peripheral tissues: (1) the
physicochemical characteristics of the compound (e.g., pK , lipid solubility, molecular volume), (2) the
a
concentration gradient between blood and tissues, (3) the ratio of blood flow to tissue mass, (4) the
relative affinity of the chemical for blood and tissue constituents, and (5) the activity of specific membrane