Great Yarmouth's economy in the Middle Ages was heavily dependent on maritime commerce, and particularly commerce in fish. This was because of its very close relationship with the sea, the town often being described as founded on a sandbank. The generally accepted view is that millions of years ago, when East Anglia emerged from the sea, its river system gradually developed, and some of these rivers converged into a broad estuary leading out to the sea for some distance on either side of the area where Yarmouth now stands. The build-up of deposits of sand, clay, and gravel in this estuary slowly formed a long spit across the estuary, later breached by run-off from the Yare, Waveney, and Bure valleys, and still later reforming, to give shape to the estuary of the last millennium.

This spit is believed to have attracted fishermen from other parts of the English coast, initially to temporary camps during the fishing season, but later as permanent settlers, perhaps as early as Late Roman times, but more likely by around 1000 A.D. – when, archaeological evidence suggests, a settlement was destroyed by fire. Despite that setback, by the time of Domesday Book Yarmouth was a small royal town with a population of perhaps 400, including a number of residents explicitly identified as fishermen. At this period the herring fishery was largely a coastal affair; not until the fifteenth century were fishing-boats venturing far out into the North Sea. Some fish could be caught in the spring, but the main fishing season was from mid-September into mid-November, when the herring shoals arrived off the coast to spawn. By the opening of the fourteenth century herring was the cheapest source of protein in the English diet – something that changed over the course of the century as fish stocks shrank, costs of doing business increased, and Yarmouth merchants attempted to monopolize profits from the industry – and was not only an important source of nutrition for the common man, but also important to the king for provisioning his armies; in 1338, for example, Edward III required 40 lasts of herring for the use of an army bound for France.

Yarmouth was the main focal point for the herring trade, supplying not only much of the English demand, but also foreign needs. The fishing season was paralleled by a herring fair, held in Yarmouth's large marketplace from Michaelmas (29 September) to Martinmas (11 November), which attracted buyers from London, France, Flanders, and as far afield as Lombardy. Swinden [p.94] tells us, based on his analysis of a Yarmouth murage roll, that over the course of five days during the1344 season 60 foreign ships entered the town's harbour to acquire herring, and other evidence suggests hundreds of ships and boats, English and foreign, visited over the course of the season.

More important participants were the townsmen of the fishing communities of the Cinque Ports, a confederation bound together in part by their common interest in the herring fishery. As part of their reward for naval service to the Crown, they were granted the right to run the herring fair, though we do not know precisely what rights this entailed prior to 1277 (when they were reduced). The competition was resented by the burgesses of Yarmouth; in 1198 the reeve of Yarmouth was claiming the authority to run a fair court, but whether this was a challenge to Ports rights at the fair is not clear. The resentment expressed itself in obstructionism, efforts to gain greater influence over trading (such as through the institution of hosting, an advantage almost monopolized by leading Yarmouth businessmen and jealously protected), and violent conflict ranging from the killing of one of the Cinque Ports bailiffs by a Yarmouth bailiff (who was hung in consequence), to outbursts of open warfare. The feuding seems to have begun in the reign of John, whose charter (1209) to Yarmouth granted basic self-government and approved a merchant gild, yet who in 1215 granted, or confirmed, that the Cinque Ports could have a court at the fair to deal with any legal disputes involving residents of their member towns. This set the scene for two rival authorities to clash: those of Yarmouth trying to assert their new jurisdictional autonomy, while the Ports sought to uphold the privileges they had obtained during fair-time.

It was not in interest of the king – particularly when he needed to unite his subjects against foreign enemies – to have his towns so at odds with each other that the peace of the realm was threatened; This was especially true when those towns were had crucial roles in supplying the realm with affordable fish and the Crown with ships. Royal efforts were made to settle the long-standing disputes between the Portsmen and the burgesses of Yarmouth, but without so disadvantaging one party that it would become unable to maintain the aforementioned roles; the parties could usually be brought to an uneasy truce, though the underlying rancour was not to be effectively dispelled. In 1277, as part of a settlement award, Edward I tried to strike a balance. He confirmed customary rights of toll-free 'den and strond', which allowed Ports fishermen to beach their boats (or otherwise moor them) and to dry and mend their nets on the Yarmouth Denes, and fixed them in a charter to the Ports the following year. The Ports retained their administration of the fair, including the employment of four sergeants to help keep the peace, and Yarmouth was prohibited from placing guards aboard ships, who could obstruct sales taking place there. But the fair court, administering merchant law, was to be presided over jointly by the wardens appointed by the Ports and by Yarmouth bailiffs, and profits from the court were to be shared (depending on the origins of the party from which taken), while a penny toll levied per ship was also to be divided, with the Ports taking a fixed amount and anything beyond that going to Yarmouth. Neither party was to take any exaction from minstrels or prostitutes servicing the fair.

However, the Ports were hardly likely to be content with a diminution of their rights, nor the Yarmouth burgesses satisfied with their gains; both sides sought to frustrate the other. In 1290 a new peace accord had to be negotiated and pardons issues, after an incident off the coast of Brittany resulted in deaths. Then in 1297, as Edward was overseeing his navy landing troops onto the Flanders coast at the mouth of the Zwin, the crews of the Cinque Ports contingent attacked those of the Yarmouth ships, killing many and setting fire to over twenty of their ships. A few years later Edward tried for a new accord, his ordinance of 1304, confirming the earlier settlement with modifications. The national political disturbances during the reign of Edward II, weakening the control of the central government, provided scope for more lawless group behaviour by Yarmouth against various rivals, while it has been suggested that assaults on two Yarmouth bailiffs in 1314 may have been the work of Portsmen, a Ports petition to parliament that year accusing Yarmouth authorities of blocking their administration of the fair. By 1316 both sides were equipping fleets to attack one another. Things seem to have been quieter during the reign of Edward III, partly because distracted by the naval dimension of the war with France, but also because the Cinque Ports turned their competitive attentions towards Southampton.

In part, the Statute of Herring marks a further effort in this process, but at a time when the supply of fish was declining and prices were rising alarmingly. The extraordinary concern to ensure adequate provision of a staple of the English diet is seen not only in that statute, but in a two others issuing from the same parliament. One statute, covering a range of topics, included a chapter prohibiting fishmongers and other victuallers of London from interfering with the right of anyone bringing victuals into the city to sell them freely to whomever they wished, and requiring the mayor and aldermen to supervise the city victualling trades in much the way they administered the assizes of bread, ale, and wine, notwithstanding any charters of privilege obtained by the victualling companies in the past.

More substantive was the other legislation, known as the Statute for Salt Fish. It was prompted by a complaint that salted fish from the northern Norfolk port of Blakeney (where there was also a fish fair), and its adjacent coasts, were being sold at excessively high prices. The statute specified that:

• Vessels known as doggers and lodeships based at Blakeney or at other communities along that coast between Blakeney and Cromer were to unload their catches only at Blakeney, and none of the cargo was to be removed to any private house until deals had been finalized during daylight hours.
• That no-one was to examine or interfere with the fish except for the buyer.
• That the quality of lob, ling, and cod put up for sale be as good (apparently in terms of size) as in the past.
• Prices of fish sold at the fair were to be set, through agreement between fishermen and merchants, at the beginning of the fair, and were to hold throughout the duration of the fair; there were to be no private sales (i.e. outside of the fair).
• No ship-owner, sailor, or anyone else was to store fish in their homes, holding them back to retail after the fair was over (in the hope of getting a higher price than that set for the fair).
• No-one in Norfolk other than the owners, masters, or crew of ships engaged in fishing were to buy nets, hooks, or other fishing equipment.

There was an effort to enforce the Statute of Herring at the herring fair of that year, when complaints by Londoners prompted investigations that led to confiscations of herring from a number of Yarmouth hosts. But in the longer-term the statute seems to have been ineffective, judging from later reiterations, and from complaints that Yarmouth hosts were ignoring its terms.

The Ordinance of Herring in 1360 recognized that the statute had cause more problems than it solved. By encouraging free trade it only pushed prices up, through enabling greater participation and competitiveness in the haggling process; while the introduction of a larger number of purchasers to each lot of herring increased the time and cost involved in the seller obtaining payments. The restriction of trading to between sunrise and sunset had proved detrimental to fishermen whose boats got back into port late in the day, forcing them to wait until the morrow (when they should have gone back out fishing) to sell their catch. Amendments were made so that a deal had to be reached between seller and first interested buyer before others could get involved until a price was fixed between that pair. And fishermen were permitted to send their catch to market whenever they arrived back at port. Still later, the effort to control prices was dropped. In the fifteenth century Yarmouth attempted self-regulation, but this was aimed at preserving the dominant role of local men and excluding outsiders from the curing industry, reinforcing the institution of hosting, while excluding alien residents from it, yet at the same time assuring the community a share of the herring catch.

Hostilities between Great Yarmouth and the Cinque Ports diminished somewhat as poorer returns from the fishery (particularly evident from the 1380s onwards), and the decline in the fortunes of the Ports communities, prompted them gradually to reduce their presence at the fair, and in the seventeenth century to abandon it altogether.

But Yarmouth's prosperity was not exclusively dependent on the fishery, it was also one of the most important of England's east coast ports, and because of that it became a centre for ship-owning and thus one of the leading sources of vessels for transporting armies and their supplies, combatting enemy fleets, and policing coastal waters. Yarmouth's decline in the Late Middle Ages would have much to do with losses to its shipping – both temporary, through prolonged periods of arrest, and permanent, through capture or destruction – during wartime, together with the high costs of maintaining an accessible harbour and the defences needed by a frontier port. In preserving its role in maritime commerce, its merchants also had to worry about rivals that were closer to home and year-round.

On the far (western) bank of the Yare, opposite Great Yarmouth, sat Little Yarmouth, also known as Southtown, and Gorleston (probably named after the Yare), accessed from Great Yarmouth by a ferry service until a bridge was built in 1417. These hamlets may have existed earlier than a permanent settlement at Great Yarmouth; the fishermen mentioned in the Domesday entry for Yarmouth actually belonged to Gorleston, which was then royal demesne. They later became annexed to a longer swath of territory known as Lothingland, in Domesday a royal manor with the status of a half-hundred.

Bounded by the sea and Lake Lothing on two sides, and the rivers Yare and Waveney on the others, Lothingland was effectively an island stretching some ten miles in length along the most north-easterly stretch of Suffolk coast. It encompassed a number of hamlets, but no market town until Lowestoft attained that status. The market that the lord of Lothingland claimed, in the legal battle with Great Yarmouth, to have by ancient right may have been that located at Lowestoft, or one at Gorleston; there is no documentary evidence of the latter until 1274/75, although we hear ca.1211 of the men of Lothingland having a market. The annexation of Gorleston to the half-hundred, probably in the early years of Edward II, may have provided a basis for the earl's officials to renew a challenge to Great Yarmouth's efforts to monopolize commercial traffic entering the haven between Great Yarmouth and Little Yarmouth/Gorleston.

This contest seems to have begun in 1228, when the warden of Lothingland, Roger fitz Osbert incensed the men of Great Yarmouth by collecting customs from ships using the haven – perhaps only those mooring off Gorleston and selling wares from on board, which was the basis of the claim of right put forward in a later court hearing. The town complained to the king that its liberties, under John's charter, had been infringed and that fitz Osbert had instituted a new market that was damaging to Great Yarmouth's, not least because he was enticing ships to unload there. The king had an enquiry held at Yarmouth in March. Its findings were that the whole haven belonged to Great Yarmouth and all great wares should be landed there, but that importers of petty goods had the option of unloading at Lothingland if they wished; it also found that the Lothingland market was not new, though its site had been changed when at threat from erosion by the sea. That same year Henry III granted the manor of Lothingland to John de Balliol, whose son, after having been nominated by Edward I as king of Scotland, made an alliance against Edward and so forfeited his English possessions. Edward gave Lothingland (1306) to his nephew John de Bretaigne, Earl of Richmond.

The judgement of 1228 may not have satisfied Great Yarmouth's burgesses, since it left its neighbours across the water with a potential market for victuals, including fish, which could cut into Great Yarmouth's revenues. In 1256 they obtained from the king a new charter granting that all goods, including fish, brought into the haven should be sold in Great Yarmouth's market, and without the intervention of any brokers (possibly meaning middlemen from Gorleston and Little Yarmouth). Yarmouth took the opportunity of the change in the lordship of Lothingland in 1306 to seek from the king clarification of its charter-given rights over collecting tolls in the haven, and the royal explication included the specification that fish were among the wares that, once brought into the haven, had to be sold at Great Yarmouth, without any intervention from broker or forestaller. A series of disputes ensued, and efforts by both parties to enforce what they considered their rights to collect tolls from commercial vessels that anchored on their side of the haven; London too objected (in the first parliament of 1327), for some of their citizens had established warehouses at Little Yarmouth and Gorleston. Great Yarmouth complained that force was used to persuade arriving ships to dock on the Gorleston side, and there are occasional documented incidents of intimidation and violence; for example, in the period 1327-30 two Yarmouth merchants, Richard Rose and Henry Randolf, were independently suing groups of Gorleston men, on allegations the latter had assaulted their servants and used force to carry off their merchandize and even Rose's boat. John Elys brought a similar charge against a Gorleston group in 1331, and the same year a death resulted from one of these acts of aggression. To an extent these kinds of self-help behaviours were a characteristic of the relative lawlessness of a frontier society; the sheriff had difficulty laying hands on any of those accused in these and other such complaints, and at times the bailiffs of Great Yarmouth themselves seem to have been deliberately obstructive to the king's justice.

In 1328 the Earl of Richmond requested the king recognize his right – which, it was subsequently dubiously alleged (dubiously but difficult to disprove), hearked back to the days of Canute and Harold – to half the waters of the haven, the right to take tolls on cargoes loaded or unloaded in that half, and the right of his tenants to trade freely through market and fair there. The burgesses responded by pointing to the explication of 1306, an inquisition held in the same year and another in 1325/26 whose decisions had favoured Great Yarmouth. The document given above was part of the subsequent drawn out court battle, whose final judgement was embodied in royal letters patent issued in July 1332, which thereafter left Great Yarmouth virtually unchallenged in its own port.

Mid-century, however, problems with that harbour silting up led Yarmouth's burgesses to petition the king to annex to the town a stretch of shoreline nearer the mouth of the haven, known as Kirkely Road, to be used for ships to unload and tolls and customs to be collected there. This was not without opposition from Lowestoft, which had benefited from ships using that landing-point to avoid customs collected at Yarmouth; the Waveney entered the sea at a point between Lowestoft and Kirkley. Lowestoft had, since the time of Domesday, grown into a thriving fishing centre that represented another threat to Yarmouth's hold on the fishery; by 1526 it would have a fishing fleet of twenty vessels and the greater part of the male population were employed by the industry. In 1372, the king acceded to the lobbying by Yarmouth and Kirkley Road was united to Yarmouth in return for a ten percent increase to the fee farm. The charter grant reiterated principles previously established: that no ships should unload cargoes of herring or other goods at anywhere within seven leagues of Great Yarmouth (now extended to include Kirkley Road), except any whose cargo belonged to the ship-owner (thus protecting Gorleston's rights), and that during the herring fair no other fair could be held anywhere within seven leagues, with revocation of any grant to the contrary made to Lowestoft (which lay within the specified radius). Within a generation the Kirkley Road location would itself be experiencing silting problems.

But Yarmouth's influence with those who governed the kingdom was already waning as its ability to meet the Crown's need for ships diminished. An enquiry in 1348 found that since 1335 Yarmouth men had lost 40 ships in naval service (both from combat and from storms), and that now only 24 ships were left in local ownership, apart from ones too old or deteriorated to be serviceable. This was a symptom and cause of the community's declining share of maritime commerce; by contrast, Lynn had largely managed to avoid its ships becoming embroiled in royal service, and the fortunes of its merchants were improving at this period. During most years of the 1340s about half the town's fleet was in the king's service for six months or more, and so inaccessible to the merchant owners [Graham Cushway, Edward III and the War at Sea: The English Navy, 1327-1377. Woodbridge: Boydell Press, 2011, p.188]; nor did the king generally compensate merchants for loss of their ships when in service. A further enquiry, in 1378, found Yarmouth impoverished and depopulated, its defences and other infrastructure in a state of disrepair, and over the next century or so the townsmen repeatedly requested financial relief from the king, sometimes citing herring trade competition from Lowestoft, Gorleston, and the Cinque Ports among reasons for its decline.

The Statute of Herring gave advance warning of a growing effort by interests opposed to Yarmouth's dominance of the fishery. This effort reached a peak at the Good Parliament of 1376 and those of the following years. At the former a leading Yarmouth merchant, William Elys,, was accused of a number of charges, including illegally extorting payments from ships of Scotland and other countries which storms forced into the sheltered waters off Kirkley Road. Two of those who gave evidence against him were John Botild and William Cooper, who had been among a group of Lowestoft men indicted for wilfully ignoring the altered status of Kirkley Road; this pair had acted as sureties for the payment demanded from the Scottish ship. Botild, a fish merchant, had previously been accused by Lyons (see below) of customs evasion (1374-76) and obstruction of collection of customs at Kirkley Road, but defended that he had paid the due customs to Elys. During his impeachment Elys counter-claimed that Botild and Cooper had tried to ambush and kill him as he was carrying to Westminster money collected as customs, though the members of his party had been able to fight them off. The accused retorted that when Elys had originally brought this charge, and secured their imprisonment on it, it was only to prevent them giving evidence before parliament, Elys having sent a message ahead to his associate, George Felbrigge, in London to have the pair arrested as soon as they arrived.

This little drama played itself out in the context of a broader political attack on the financial corruption of the caretaker government of John of Gaunt. It was almost a foregone conclusion that Elys, too closely linked with the Gaunt administration, would be found guilty. While his accusers were awarded a sum in damages, Elys languished in the Tower for some six months, until Gaunt regained some of his influence and began to reverse the accomplishments of the Good Parliament. Elys was released on the surety of ten men, of whom eight were Londoners, but was not issued a pardon and restored to his customs post until 1377, following more thorough investigations into the various charges against him.

While ostensibly the attack on Elys was fall-out from the impeachment of customs farmer Richard Lyons, of whom Elys was for a while deputy as a collector of wool customs, it can also be seen as part of the campaign to break Yarmouth's hold on the herring fishery. Following a public outcry due to a national shortage of herring, it was complained in the 1376 parliament, through a petition submitted by nine counties of eastern, southern, and midland England that Yarmouth's annexation of Kirkley Road was counter to the best interests of the realm; Yarmouth's opponents were able to force the repeal of the royal charter of 1372 and to have its herring fair discontinued, with much of the business transferring to Lowestoft and other locations. They argued that centralizing the herring trade on Yarmouth had been detrimental, because local merchants' control over sales had discouraged many foreign fishermen from bringing their catches to Yarmouth, and that the town was not a good distribution centre for the rest of the country because the rivers encompassing Yarmouth were a hindrance to pedlars, carts, and packhorses coming from southern and Midlands counties to buy herring to provision their home localities; all of which Yarmouth denied.

A royal commission to look at the whole matter more closely lapsed with Edward III's death, but was revived in early 1378, under Gaunt's caretaker government. Lowestoft and Yarmouth subsequently presented their cases to the king's council and, at the parliament of 1378, Yarmouth followed up with a petition. This argued that, as a coastal community on a very confined site, it relied on the herring trade for revenues to maintain itself as a fortified coastal defence and to pay its fee-farm and taxes; and that the cancellation of Yarmouth's privileged role in the trade had only pushed prices higher and curing quality lower. This position was supported by the report of a commssion of enquiry headed by the earl of Suffolk. Despite unfavourable sentiment in the parliament to both Yarmouth and Gaunt, the latter had the repeal reversed; when the news was proclaimed at Lowestoft there was a riot. The matter continued to be a political football until Richard II's charter to Yarmouth (1385) reconfirmed Yarmouth's advantages, and a chancery battle early in Henry IV's reign persuaded the men of Lowestoft to give up a lost cause.

Great Yarmouth's struggle against various rival groups to defend and enforce its commercial advantages was a recurrent issue to be addressed by the royal justice system and by parliament. To the historian, for whom the passage of time too often appears compressed, it seems that Yarmouth's various problems – not just the rivalries, but its decaying defences, the periodic need to relocate its harbour, its shrinking fleet, its growing complaints of inability to pay its financial dues, and piratic acts committed by or against its ships – together with popular resentment over the town's influence on the herring market, must have made it something of a nuisance. If so, there is little sign in the official record of royal exasperation. Except when matters were caught up in larger political contests, the king made some effort to strike a balance between the various interests at stake. But compromises did not satisfy any of the contesting parties, which were often prepared to resort to extra-legal means to uphold what they saw as their rights.