HOBSON ET AL.: CREPUSCULAR AND NOCTURNAL ACTIVITIES OF CALIFORNIA FISHES 



Determining Activity Patterns in Fishes 



Our accounts of activity patterns in the fishes 

 stem from direct underwater observations and 

 from study of gut contents. The underwater obser- 

 vations were made using scuba and by snorkehng 

 during all hours of day and night. The gut contents 

 were from fishes speared at all hours of day and 

 night, but primarily during late afternoon and 

 within 2 h before first morning light — times 

 which best distinguish diurnal and nocturnal 

 habits. To study the gut contents, the digestive 

 tract of each fish specimen was removed im- 

 mediately after collection and preserved in a 10% 

 formaldehyde solution. Analysis under a binocu- 

 lar dissecting scope was performed later in the 

 laboratory. We note in this report only major food 

 items that we believe might add insight to our 

 accounts of diel activity patterns. More detailed 

 accounts of the food habits are given elsewhere 

 (Hobson and Chess 1976; in prep.). All mea- 

 surements offish size are of standard length (SL). 

 Although our accounts center on crepuscular and 

 nocturnal events, we describe enough of what 

 happens in daylight to consider these events in the 

 context of diel patterns. 



Determining Spectral Photosensitivity 

 of Fishes 



each retinal extract was homogeneous or con- 

 tained more than one visual pigment. Pigment 

 analysis was assisted by a computer program 

 (Munz and Allen 1968) designed to test for 

 homogeneity and also to characterize each visual 

 pigment by estimation of the wavelength of peak 

 absorbance (Amax)- Generally, the major photo- 

 labile component in a vertebrate retinal extract is 

 the rod visual pigment, and the minor compo- 

 nent(s) is the cone visual pigment(s) (Munz and 

 McFarland 1975; McFarland and Munz 1975b). 

 Thus, in each retinal extract from the Catalina 

 samples the dominant pigment is considered the 

 scotopic (or rod) visual pigment. 



UNDERWATER PHOTIC ENVIRONMENT 



Coastal waters characteristically absorb light of 

 shorter wavelengths than do oceanic waters be- 

 cause they contain more dissolved organic matter. 

 They also scatter more light due to higher con- 

 centrations of suspended particulate matter. As a 

 result, they transmit light of longer wavelengths, 

 and, therefore, under a midday sun appear blue- 

 green, rather than blue like the open sea (see Jer- 

 lov 1968 for classification of water tj^es). Starting 

 with these well-established facts, we attempted to 

 characterize the underwater photic environment 

 at Santa Catalina Island. 



Two techniques were used to obtain both fresh, 

 and dark-adapted retinae from the fishes. Some of 

 the specimens were captured alive and dark- 

 adapted under laboratory conditions, whereas 

 others were speared at night and immediately 

 placed in dark containers before being returned to 

 the laboratory for additional treatment. Spectral 

 absorbance characteristics of the visual pigments 

 from these retinae were determined by standard 

 procedures. After each fish was dark-adapted, its 

 eyes were enucleated, the retinae were removed 

 under deep red light (Wratten #2 filter), and then 

 frozen in 4% alum. Later the retinae were thawed, 

 washed in triplicate, centrifuged, and the pellet 

 extracted in 2% digitonin. Sonication of the pellet 

 at 0° C for 1 min assisted solubilization of the 

 visual pigment. After centrifugation, 10% by vol- 

 ume of saturated sodium borate and 10% by vol- 

 ume of 0.2 M hydroxylamine were added to the 

 supernatant and the spectral absorbance of the 

 extracted visual pigment recorded with a Cary 14 

 spectrophotometer. The method of partial bleach- 

 ing (Dartnall 1952) was applied to test whether 



Submarine Daylight 

 Midday Spectra 



Essentially all submarine daylight meaningful 

 to fishes is produced by the sun. Although photic 

 conditions during midday are not our concern in 

 this paper (they will be considered in a later re- 

 port), midday spectra effectively illustrate some 

 fundamental aspects of the photic environment 

 that are needed to understand scotopic vision in 

 fishes. In particular, midday spectra can be used to 

 define the spectral transmission characteristics of 

 a given water mass, and so provide means to com- 

 pare the photic environment in Californian coast- 

 al waters with the photic conditions elsewhere, 

 including comparisons of crepuscular and noctur- 

 nal circumstances between different habitats. 



Californian coastal waters vary greatly in the 

 way they transmit light, and while some of this 

 variation is seasonal, much is shorter term and 

 irregular. At times during our study at Santa 

 Catalina very little suspended material was pres- 



