Fisheries Technologist cheat sheet

Sonar equation: SL − 2TL + TS − (NL − DI) = DT; echo detection requires the received level to exceed detection threshold after noise is reduced by directivity index.

Acoustic target strength scales with fish size: TS ≈ 19.1 log10(L) − 0.9 log10(f) − 62 for swimbladder fish (L in cm, f in kHz), so doubling length raises TS by roughly 5.7 dB.

Echosounder volume backscattering sv (m⁻¹) converts to areal density via sa = sv × cτψ/2, where τ is pulse duration, c sound speed (~1500 m/s), ψ the equivalent beam angle.

Fish density from acoustics: ρ = sa / σbs, with σbs = 4π·10^(TS/10); a single large target contributes disproportionately, so TS distributions must be inspected.

Hydroacoustic frequency choice: low frequency (38 kHz) gives wider beam and greater range; high frequency (120–200 kHz) gives better resolution of small targets but shorter range and more attenuation.

CPUE is a relative abundance index only if catchability q is constant: N̂ = CPUE/q; hyperstability occurs when q rises with abundance, masking declines in stock size.

Schaefer surplus production model: dB/dt = rB(1 − B/K) − C; at equilibrium MSY = rK/4, occurring at B = K/2.

Gear selectivity described by logistic curve P(L) = 1/(1 + exp[−S(L − L50)]); L50 is the length at 50% retention and S the steepness of selection.

Length–weight relationship W = aL^b; b ≈ 3 indicates isometric growth, b > 3 positive allometry; parameters are fitted by log-linear regression of ln W on ln L.

Dissolved oxygen is the first limiting factor in intensive systems; warm water holds less O2, so safe DO thresholds fall as temperature rises (e.g., tilapia tolerate lower DO than salmonids).

Unionized ammonia (NH3) is the toxic fraction; its share of total ammonia nitrogen rises with pH and temperature, so toxicity worsens at high pH even when TAN is unchanged.

Nitrogen cycle in biofilters: Nitrosomonas-type bacteria oxidize NH4+ to NO2−; Nitrobacter/Nitrospira oxidize NO2− to NO3−; nitrite peaks during biofilter startup.

Stocking density rule for flow-through systems: allowable biomass depends on inflow rate and oxygen budget; feed input is limited by the oxygen consumed per kg feed, not by tank volume alone.

Feed conversion ratio FCR = feed offered / wet weight gain; apparent FCR below ~1.0 usually signals measurement error or inclusion of natural food.

Protein requirement falls with fish size and rises with water temperature up to the optimum; excess dietary protein is catabolized and excreted as ammonia, raising biofilter load.

Carrying capacity of a pond is set by natural productivity plus aeration and feed; aeration sustains higher standing crops but cannot exceed the oxygen transfer capacity of the aerators.

Sex reversal in tilapia is achieved by feeding 17α-methyltestosterone during the undifferentiated fry stage (roughly first 21–28 days post-hatch), yielding predominantly male populations.

Recirculating aquaculture system (RAS) water reuse requires solids removal, biofiltration, and gas exchange; water exchange of a few percent per day is typical, with salinity and alkalinity monitored.

Trawls, seines, and gillnets are active/passive gear classes with different selectivity; gillnet selectivity depends on mesh size relative to fish girth, producing a bell-shaped retention curve rather than a logistic one.

Longline catch rates depend on hook size, bait, and soak time; soak-time saturation causes CPUE to plateau, so effort must be standardized to a fixed soak duration.

FAD (fish aggregating device) sets on purse seiners raise catch of small tunas and bycatch; management often restricts FAD closures to protect juvenile stocks.

Fishing mortality F and natural mortality M combine into total mortality Z = F + M; estimated from catch curves as the descending slope of ln(catch) versus age.

Virtual population analysis (VPA) back-calculates cohort size from catch-at-age and M; terminal F assumptions strongly influence recent-year estimates.

Maximum sustainable yield from the Beverton–Holt yield-per-recruit model depends on F and the age at first capture; raising mesh size increases yield per recruit if growth overfishing exists.

Catch per unit effort standardization uses generalized linear models (e.g., log-link, delta-lognormal) to remove effects of vessel, area, season, and gear before using CPUE as an index.

Bycatch reduction devices (BRDs) such as turtle excluder devices and square-mesh panels reduce non-target catch while retaining marketable size classes of the target species.

Rigor mortis in fish proceeds through pre-rigor, rigor, and resolution phases; handling during rigor causes gaping and texture damage, so processing should occur pre-rigor or post-resolution.

Chilling rate governs shelf life: fish should reach 0 °C in ice as fast as possible; every 10 °C rise roughly doubles bacterial spoilage rate (Q10 ≈ 2).

Total volatile basic nitrogen (TVB-N) and trimethylamine (TMA) are spoilage indicators; TVB-N limits around 25–35 mg/100 g are used as rejection thresholds in many standards.

Water activity and salt content control microbial growth in cured fish; drying to aw below ~0.75 inhibits most spoilage bacteria but not all xerophilic molds.

Freezing preserves quality best at −18 °C or below; slow freezing forms large extracellular ice crystals that rupture cells and cause drip loss on thawing.

Histamine (scombroid toxin) forms in scombroid fish by bacterial decarboxylation of histidine when fish are held above ~4 °C; it is heat-stable and not destroyed by cooking.

Cold chain management traces temperature from harvest to retail; time–temperature integrators and data loggers document abuse, and HACCP critical control points are set at chilling and storage steps.

Yield and value addition: fillet yield = fillet weight / whole fish weight; species with high fillet yield and low processing waste give better economic returns in value-added products.

Traceability and eco-certification schemes require chain-of-custody records linking catch or farm lot to the final product, supporting food safety and sustainability claims.

Acoustic backscattering cross-section σbs = 4π·10^(TS/10); converting TS to density requires this linear form, not the decibel value.

Catchability q links observed CPUE to true abundance: CPUE = q·N; changes in q from gear, technology, or fisher behavior bias the index.

Surplus production MSY = rK/4 at B = K/2, derived by setting dB/dt = 0 and maximizing the Schaefer production function.

Length-based growth: von Bertalanffy Lt = L∞(1 − e^(−K(t − t0))); K is the growth coefficient and t0 the theoretical age at zero length.

Gear selectivity logistic: L50 marks 50% retention; mesh size and codend design shift L50 and thus the size at first capture.

Sonar equation terms: SL source level, TL transmission loss, TS target strength, NL noise level, DI directivity index, DT detection threshold.

Areal backscattering sa = sv·cτψ/2 converts volume backscattering to a density index per unit area for stock assessment.

Unionized NH3 fraction rises with pH and temperature; total ammonia nitrogen is not the toxic measure, so toxicity is assessed at the NH3 fraction.

Biofilter startup: ammonia-oxidizing bacteria establish before nitrite-oxidizing bacteria, producing a transient nitrite peak.

FCR = feed offered / wet weight gain; values near or below 1.0 usually indicate measurement error or unaccounted natural food.

Tilapia sex reversal uses 17α-methyltestosterone feeding during the undifferentiated fry stage, roughly the first 21–28 days post-hatch.

Dissolved oxygen is the first limiting factor in intensive systems, and warm water holds less oxygen, so safe DO thresholds fall as temperature rises.

RAS reuse requires solids removal, biofiltration, and gas exchange, with water exchange typically a few percent per day.

Excess dietary protein is catabolized and excreted as ammonia, increasing biofilter load; protein requirement falls with fish size.

Total mortality Z = F + M, estimated as the descending slope of ln(catch) versus age in a catch curve.

Gillnet selectivity is bell-shaped because retention depends on fish girth relative to mesh size, unlike the logistic curve of trawl codends.

Longline soak-time saturation makes CPUE plateau, so effort must be standardized to a fixed soak duration.

VPA back-calculates cohort size from catch-at-age and M, and terminal F assumptions strongly influence recent-year estimates.

Yield-per-recruit depends on F and age at first capture; larger mesh raises yield per recruit when growth overfishing exists.

CPUE standardization uses generalized linear models such as log-link delta-lognormal to remove vessel, area, season, and gear effects.

FAD sets on purse seiners raise catch of small tunas and bycatch, so FAD closures are used to protect juvenile stocks.

Q10 ≈ 2 means every 10 °C rise roughly doubles bacterial spoilage rate, so rapid chilling to 0 °C extends shelf life.

TVB-N and TMA are spoilage indicators; TVB-N limits around 25–35 mg/100 g are used as rejection thresholds in many standards.

Histamine is heat-stable and forms in scombroid fish held above ~4 °C by bacterial decarboxylation of histidine.

Freezing at −18 °C or below limits quality loss; slow freezing forms large extracellular ice crystals and causes drip loss on thawing.

Handling during rigor mortis causes gaping and texture damage, so processing should occur pre-rigor or after resolution.

Drying to water activity below ~0.75 inhibits most spoilage bacteria but not all xerophilic molds.

Fillet yield = fillet weight / whole fish weight, a key economic metric for value-added processing.

TS ≈ 19.1 log10(L) − 0.9 log10(f) − 62 for swimbladder fish, so doubling length raises target strength by roughly 5.7 dB.

Echosounder frequency trade-off: 38 kHz gives wider beam and greater range, while 120–200 kHz gives better resolution but shorter range.

Hyperstability occurs when catchability rises with abundance, masking stock declines in CPUE series.

Schaefer model MSY = rK/4 at B = K/2.

Length–weight W = aL^b; b ≈ 3 indicates isometric growth, b > 3 positive allometry.

Acoustic density ρ = sa / σbs, with σbs = 4π·10^(TS/10).

Sonar equation balances source level, transmission loss, target strength, noise, directivity, and detection threshold.

Tilapia tolerate lower dissolved oxygen than salmonids, so safe DO thresholds are species-specific.

Nitrite peaks during biofilter startup because ammonia oxidizers establish before nitrite oxidizers.

Carrying capacity is set by natural productivity plus aeration and feed; aeration cannot exceed the oxygen transfer capacity of the aerators.

Protein requirement falls with fish size and rises with temperature up to the optimum.

FCR = feed offered / wet weight gain.

RAS water exchange of a few percent per day is typical, with salinity and alkalinity monitored.

Unionized ammonia toxicity worsens at high pH even when total ammonia nitrogen is unchanged.

Catch curve slope of ln(catch) versus age gives Z = F + M.

Beverton–Holt yield-per-recruit depends on F and age at first capture.

Gillnet retention is bell-shaped, while trawl codend selectivity is logistic.

CPUE standardization removes vessel, area, season, and gear effects using generalized linear models.

VPA terminal F assumptions strongly influence recent-year estimates.

Longline soak-time saturation causes CPUE to plateau.

BRDs such as turtle excluder devices and square-mesh panels reduce non-target catch.

Scombroid histamine is heat-stable and not destroyed by cooking.

TVB-N limits around 25–35 mg/100 g are used as rejection thresholds.

Q10 ≈ 2 for bacterial spoilage means a 10 °C rise doubles spoilage rate.

Water activity below ~0.75 inhibits most spoilage bacteria but not all xerophilic molds.

Freezing at −18 °C or below minimizes quality loss; slow freezing causes drip loss.

Fillet yield = fillet weight / whole fish weight.

HACCP critical control points are set at chilling and storage steps in the cold chain.