Fish in the Bay – Special Report, Color Change Experiment #7

Belated report:  This Clupeiform color change experiment was conducted in December but never reported.  This is part of my longstanding interest in investigating salinity-induced color change in Clupeiform fishes.

Clupeiformes is the order of fishes that includes Herring, Anchovies, Sardines, and Shad.  In recent years of trawling surveys, we have observed that dorsal sides of Clupeiform fishes tend to be blue in salty water and brown or gold in fresher brackish water.  Starting in 2020, a series of experiments were carried out to determine if there might be a causative relationship.

Why report now?  I am archiving old photo files to the UC Davis OG Fish Lab.  I suddenly realized that undocumented experiments contaminate the database.  Shad and Anchovies of various colors no longer matched ambient salinities at trawling stations where I photographed my impromptu tests.  All experiments must be documented.


Shad and Anchovies were photographed at stations Dmp1 and UCoy1.  They were then emersed in fresh water and then back to salt water and photographed over minutes.


 1. Experiment 7, Trial #1 at station Dmp1.

Simple experiment:  For the first trial, five bluish Anchovies and 2 greenish American Shad were taken from brackish slough water.  Salinity was 17.7 ppt according to the YSI meter.  

  • The fish were emersed in unchlorinated fresh water (0 ppt).
  • Multiple photos were taken per minute as the fish acclimated to lower salinity.
  • Both Shad and Anchovies began conspicuously ‘browning-down’ within the first minute.


Browning was practically complete in Shad within about three minutes.  Anchovies took a minute or two longer.


Next question: would they blue-up again when exposed to high salinity?


For the second phase of this trial, the Anchovies and shad were reintroduced to salty slough water.  By this time, we had motored downstream in Dump Slough to station Dmp2.  Salinity of the slough water was slightly higher at 23.7.   

  • The fish were emersed in salty water from Dmp2 (23.7 ppt).
  • Multiple photos were taken per minute as the fish blued-up.
  • Both Shad and Anchovies began conspicuously ‘blueing-up’ within the first minute.


Shad substantially blued-up within 3 minutes. Anchovies again took an extra minute or two.


It was hard to tell if Anchovies are slower or if melanophore darkness was simply making it harder to see the color change.


Traces of brownness remained in both Shad and Anchovies after several minutes. But, sufficient information had been gleaned.  We must not allow our curiosity to run amok!


The blue fishes were released back to the slough.


 2. Experiment 7, Trial #2 at station UCoy1.

A second trial was conducted on another Anchovy at UCoy1.  Slough salinity was 21.7 ppt, and the Anchovy was blue when caught. 

  • The Anchovy’s dorsal side changed from blue to green roughly one minute after emersion in fresh water.


The Anchovy browned-down to brown with green splotches within six minutes.  After 10 minutes the Anchovy was essentially totally brown.


Second phase:  The fish was reintroduced to salty water (21.7 ppt).  Blueness returned within minutes! 

  • Anchovy was released alive and with no apparent ill effects.


Herring Experiment.  A reminder:  We conducted a color-change experiment on a Pacific Herring in February 2021 with essentially the same results.  The Herring browned-down in fresh water and then blued-up in salt within minutes after emersion.

  • Reproducibility appears to be excellent across multiple species. Perhaps the hypothesis becomes a theory???



“I could be wrong.  I could be right.”

Public Image Inc.

Comments are closed.