100+ Free Fisheries Technologist Practice Questions

Explanation: Dissolved oxygen (DO) is the most critical and frequently limiting water quality parameter in intensive aquaculture systems because fish and other aquatic organisms depend on it for aerobic respiration. Depletion of DO can lead to rapid mortality, reduced feed intake, and increased susceptibility to disease. While temperature, salinity, and turbidity are important, they rarely fluctuate as rapidly or cause as immediate catastrophic loss as sudden DO depletion.

Explanation: In aquatic environments, total ammonia nitrogen exists in two forms: unionized ammonia (NH3, toxic) and ionized ammonium (NH4+, non-toxic). The equilibrium shifts toward the toxic unionized NH3 form as pH and temperature increase. At high pH, there are fewer hydrogen ions to protonate NH3 into NH4+, and higher temperatures further accelerate this chemical shift, increasing the toxicity risk for aquatic life.

Explanation: The Feed Conversion Ratio (FCR) is calculated by dividing the total feed consumed by the net biomass weight gained (FCR = Feed Given / Weight Gain). In this scenario, FCR = 1,200 kg / 800 kg = 1.50. This means 1.5 kg of feed was required to produce 1.0 kg of milkfish biomass. An FCR of 1.50 is typical for semi-intensive brackishwater milkfish systems.

Explanation: One hectare (ha) is equal to 10,000 square meters. Therefore, a 1.5-hectare pond has an area of 15,000 square meters (1.5 x 10,000). At a stocking density of 5 fish per square meter, the total number of fingerlings required is 15,000 sq m x 5 fish/sq m = 75,000 fingerlings.

Explanation: Female Nile tilapia (Oreochromis niloticus) practice maternal mouthbrooding, where the female gathers the fertilized eggs into her mouth for incubation and protection. During this brooding period, which lasts 1-2 weeks, she abstains from feeding until the fry absorb their yolk sacs and swim freely. This strategy significantly increases offspring survival rates compared to substrate-nesting species with no care.

Explanation: The larval development of Penaeus monodon consists of four distinct stages: Nauplius (non-feeding, relies on yolk), Zoea (starts feeding on phytoplankton), Mysis (resembles tiny shrimp, feeds on zooplankton), and Postlarva (settles to benthic habitats and feeds on detritus and artificial microdiets). Recognizing these stages is vital for hatchery technicians to adjust feeding regimes and water quality management.

Explanation: White Spot Syndrome Virus (WSSV) is a dsDNA virus that targets tissues of ectodermal and mesodermal origin. The cuticular hypodermis, gill epithelium, connective tissues, and hematopoietic tissues undergo severe necrosis, characterized by hypertrophied nuclei and eosinophilic to basophilic intranuclear inclusion bodies. The endodermal hepatopancreas is generally spared, which is an important diagnostic differentiator in histology.

Explanation: Specific Growth Rate (SGR) is calculated using the formula: SGR = [(ln(W_final) - ln(W_initial)) / days] x 100. Substituting the values: SGR = [(ln(50) - ln(10)) / 40] x 100 = [(3.912 - 2.303) / 40] x 100 = [1.609 / 40] x 100 = 0.0402 x 100 = 4.02% per day.

Explanation: In a Recirculating Aquaculture System (RAS), the biofilter utilizes aerobic nitrifying bacteria to detoxify excreted ammonia. Nitrosomonas bacteria oxidize ammonia (NH3) to nitrite (NO2-), which is also highly toxic. Then, Nitrobacter bacteria oxidize nitrite to nitrate (NO3-), which is relatively non-toxic to fish and tolerated at higher concentrations. This aerobic process requires high dissolved oxygen and sufficient alkalinity.

Explanation: To promote the growth of beneficial microalgae, particularly diatoms (which are favored by milkfish feeding on benthic mat 'lablab'), an N:P ratio of 10:1 to 16:1 is recommended. This ratio mirrors the Redfield ratio of marine organic matter. A lower N:P ratio often triggers blooms of cyanobacteria (blue-green algae), which can cause off-flavor issues, gill blockage, and sudden die-offs.